Congenital forms of hypertrichosis are caused by genetic mutations, and are extremely rare, unlike acquired forms. Congenital hypertrichosis is always present at birth.

- Hypertrichosis lanuginosa

Congenital hypertrichosis lanuginosa is noticeable at birth, with the infant completely covered in thin lanugo hair. In normal circumstances, lanugo hair is shed before birth and replaced by vellus hair; however, in a person with congenital hypertrichosis lanuginosa, the lanugo hair remains after birth. The palms of the hands, soles of the feet, and mucous membranes are not affected. As the person ages, the lanugo hair may thin, leaving only limited areas of hypertrichosis.

- Generalized hypertrichosis

Congenital generalized hypertrichosis causes males to exhibit excessive facial and upper body hair, whereas women exhibit less severe asymmetrical hair distribution. The palms, soles, and mucous membranes are not affected.

- Terminal hypertrichosis

Congenital terminal hypertrichosis is characterized by the presence of fully pigmented terminal hair that covers the entire body. This condition is usually accompanied by gingival hyperplasia. This form is most responsible for the term "werewolf syndrome" because of the thick, dark hair that appears. People with this condition are sometimes performers at circuses because of their unusual appearance.

- Circumscribed hypertrichosis

Congenital circumscribed hypertrichosis is associated with the presence of thick vellus hair on the upper extremities. Circumscribed signifies this type of hypertrichosis is restricted to certain parts of the body, in this case, the extensor surfaces of the upper extremities. Hairy elbow syndrome, a type of congenital circumscribed hypertrichosis, shows excessive growth on and around the elbows. This type of hypertrichosis is present at birth, becomes more prominent during aging, and regresses at puberty.

- Localized hypertrichosis

Congenital localized hypertrichosis is a localized increase in hair density and length.

- Nevoid hypertrichosis

Nevoid hypertrichosis may be present at birth or appear later in life. It features an isolated area of excessive terminal hair and is usually not related to any other diseases.

Nevoid hypertrichosis is a cutaneous condition characterized by the growth of terminal hairs in a circumscribed area.

Acquired hypertrichosis appears after birth. The multiple causes include the side effects of drugs, associations with cancer, and possible links with eating disorders. Acquired forms can usually be reduced with various treatments.

- Hypertrichosis lanuginosa

Acquired hypertrichosis lanuginosa is characterized by rapid growth of lanugo hair, particularly on the face. Hair also appears on the trunk and armpits, while palms and soles are unaffected. The excess hair is commonly referred to as malignant down. This hair is very fine and unpigmented.

- Generalized hypertrichosis

Acquired generalized hypertrichosis commonly affects the cheeks, upper lip, and chin. This form also affects the forearms and legs, but is less common in these areas. Another deformity associated with acquired generalized hypertrichosis is multiple hairs occupying the same follicle. It may also include abnormal hair growth patterns as what happens to the eyelashes in a condition known as trichiasis. Oral minoxidil treatments for hypertension are known to cause this condition. Topical minoxidil used for alopecia causes hair growth in the areas where it is applied, however this hair disappears shortly after discontinuing the use of topical minoxidil.

- Patterned hypertrichosis

Acquired patterned hypertrichosis is an increase in hair growth in a pattern formation. It is similar to acquired generalized hypertrichosis and is a sign of internal malignancy.

- Localized hypertrichosis

Acquired localized hypertrichosis is an increase in hair density and length often secondary to irritation or trauma. This form is restricted to certain areas of the body.

Hypotrichosis ("" + "" + "") is a condition of abnormal hair patterns, predominantly loss or reduction. It occurs, most frequently, by the growth of vellus hair in areas of the body that normally produce terminal hair. Typically, the individual's hair growth is normal after birth, but shortly thereafter the hair is shed and replaced with sparse, abnormal hair growth. The new hair is typically fine, short and brittle, and may lack pigmentation. Baldness may be present by the time the subject is 25 years old.

Hypotrichosis is a common feature of Hallermann–Streiff syndrome as well as others. It can also be used to describe the lack of hair growth due to chemotherapy.

The opposite of hypotrichosis is hypertrichosis, where terminal hair (thick) grows in areas that would otherwise normally have vellus hair (thin), for example abnormally thick facial hair growth in women.

Hair diseases are disorders primarily associated with the follicles of the hair.

A few examples are

- Alopecia

- Bubble hair deformity

- Hair casts

- Hair loss

- hypertrichosis

- Ingrown hair

- Monilethrix

- Premature greying of hair

- Pattern hair loss

- Trichorrhexis invaginata

Many hair diseases can be associated with distinct underlying disorders.

Piedra are fungal diseases.

Hair disease may refer to excessive shedding or baldness (or both). Balding can be localised or diffuse, scarring or non-scarring. Increased hair can be due to hormonal factors (hirsutism) or non-hormonal (hypertrichosis). Scalp disorders may or may not be associated with hair loss.

Nevoid melanoma is a cutaneous condition that may resemble a Spitz nevus or an acquired or congenital melanocytic nevus.

Prepubertal hypertrichosis is a cutaneous condition characterized by increased hair growth, and is a relatively common finding in otherwise healthy infants and children, most often occurring in individuals of Mediterranean or South Asian descent.

Becker's nevus (also known as "Becker's melanosis", "Becker's pigmentary hamartoma", "nevoid melanosis", and "pigmented hairy epidermal nevus") is a skin disorder predominantly affecting males. The nevus can be present at birth, but more often shows up around puberty. It generally first appears as an irregular pigmentation (melanosis or hyperpigmentation) on the torso or upper arm (though other areas of the body can be affected), and gradually enlarges irregularly, becoming thickened and often hairy (hypertrichosis). The nevus is due to an overgrowth of the epidermis, pigment cells (melanocytes), and hair follicles. This form of nevus was first documented in 1948 by American dermatologist Samuel William Becker (1894–1964).

Hairy palms and soles are both a type of cutaneous condition characterized by a hereditary hypertrichosis affecting the palms and soles. This condition is inherited in an autosomal dominant fashion.

Genetic forms of localized autosomal recessive hypotrichosis include:

Linear and whorled nevoid hypermelanosis (also known as "Linear nevoid hyperpigmentation," "Progressive cribriform and zosteriform hyperpigmentation," "Reticulate and zosteriform hyperpigmentation," "Reticulate hyperpigmentation of Iijima and Naito and Uyeno," "Zebra-like hyperpigmentation in whorls and streaks," and "Zebra-line hyperpigmentation") is a disorder of pigmentation that develops within a few weeks of birth and progresses for one to two years before stabilizing. There is linear and whorled hyperpigmentation following the lines of Blaschko without preceding bullae or verrucous lesions. It is important to exclude other pigmentary disorders following the Blaschko lines before making a diagnosis of linear and whorled nevoid hypermelanosis.The differential diagnoses include incontinentia pigmenti, linear epidermal nevus, hypomelanosis of Ito and Goltz syndrome. Recently, a case of linear and whorled nevoid hypermelanosis was reported in a Malaysian Chinese girl.

Hypertrichosis cubiti (also known as "hairy elbow syndrome") is a cutaneous condition characterized by multiple terminal hairs on both elbows in children.

Congenital smooth muscle hamartoma is typically a skin colored or lightly pigmented patch or plaque with hypertrichosis.

X-linked hypertrichosis is a hereditary disorders characterized by generalized congenital hypertrichosis.

One known cause of hypertrichosis cubiti is Wiedemann-Steiner syndrome.

Ear hair generally refers to the terminal hair arising from follicles inside the external auditory meatus in humans. In its broader sense, "ear hair" may also include the fine vellus hair covering much of the ear, particularly at the prominent parts of the anterior ear, or even the abnormal hair growth as seen in hypertrichosis and hirsutism. Medical research on the function of ear hair is currently very scarce.

Hair growth within the ear canal is often observed to increase in older men, together with increased growth of nose hair. Visible hair that protrudes from the ear canal is sometimes trimmed for cosmetic reasons. Excessive hair growth within or on the ear is known medically as "auricular hypertrichosis". Some men, particularly in the male population of India, have coarse hair growth along the lower portion of the helix, a condition referred to as "having "hairy pinnae"" ("hypertrichosis lanuginosa acquisita").

The congenital melanocytic nevus appears as a circumscribed, light brown to black patch or plaque, potentially very heterogeneous in consistency, covering any size surface area and any part of the body.

As compared with a melanocytic nevus, congenital melanocytic nevi are usually larger in diameter and may have excess terminal hair, a condition called hypertrichosis. If over 40 cm projected adult diameter with hypertrichosis, it is sometimes called giant hairy nevus; more usually these largest forms are known as large or giant congenital melanocytic nevus. The estimated prevalence for the largest forms is 0.002% of births.

Melanocytic Nevi often grow proportionally to the body size as the child matures. As they mature, they often develop thickness, and become elevated, although these features can also be present from birth. Prominent terminal hairs often form, especially after puberty. With maturity, the nevus can have variation in color, and the surface might be textured with proliferative growths.

Neurocutaneous melanosis is associated with the presence of either giant congenital melanocytic nevi or non-giant nevi of the skin. It is estimated that neurocutaneous melanosis is present in 2% to 45% of patients with giant congenital melanocytic nevi. Neurocutaneous melanosis is characterized by the presence of congenital melanocytic nevi on the skin and melanocytic tumors in the leptomeninges of the central nervous system.

Congenital melanocytic nevi may be divided into the following types:

- "Small-sized congenital melanocytic nevus" is defined as having a diameter less than 2 cm.

- "Medium-sized congenital melanocytic nevus" is defined as having a diameter more than 2 cm but less than 20 cm.

- "Giant congenital melanocytic nevus" (also known as "Bathing trunk nevus," "Garment nevus," "Giant hairy nevus", and "Nevus pigmentosus et pilosus") is defined by one or more large, darkly pigmented and sometimes hairy patches.

Hypertrichosis simplex of the scalp is a cutaneous condition caused by defects in the corneodesmosin protein.

Auricular hypertrichosis ("hypertrichosis lanuginosa acquisita", "hypertrichosis pinnae auris") is a genetic condition expressed as long and strong hairs growing from the helix of the pinna.

A 1991 report documented the cases of nine patients with both Becker's nevus and malignant melanoma. Of the nine melanomas, five were in the same body area as the Becker's nevus, with only one occurring within the nevus itself. As this was apparently the first documented co-occurrence of the two diseases, there is so far no evidence of higher malignancy rates in Becker's nevi versus normal skin. Nonetheless, as with any abnormal skin growth, the nevus should be monitored regularly and any sudden changes in appearance brought to the attention of one's doctor.

Gingival fibromatosis with hypertrichosis is a cutaneous condition characterized by dark terminal hairs on the peripheral face, central back, and extremities. It is a RASopathy.

Oliver–McFarlane syndrome is a condition characterized by hypertrichosis of the eyebrows and eyelashes.

Many conditions affect the human integumentary system—the organ system covering the entire surface of the body and composed of skin, hair, nails, and related muscle and glands. The major function of this system is as a barrier against the external environment. The skin weighs an average of four kilograms, covers an area of two square meters, and is made of three distinct layers: the epidermis, dermis, and subcutaneous tissue. The two main types of human skin are: glabrous skin, the hairless skin on the palms and soles (also referred to as the "palmoplantar" surfaces), and hair-bearing skin. Within the latter type, the hairs occur in structures called pilosebaceous units, each with hair follicle, sebaceous gland, and associated arrector pili muscle. In the embryo, the epidermis, hair, and glands form from the ectoderm, which is chemically influenced by the underlying mesoderm that forms the dermis and subcutaneous tissues.

The epidermis is the most superficial layer of skin, a squamous epithelium with several strata: the stratum corneum, stratum lucidum, stratum granulosum, stratum spinosum, and stratum basale. Nourishment is provided to these layers by diffusion from the dermis, since the epidermis is without direct blood supply. The epidermis contains four cell types: keratinocytes, melanocytes, Langerhans cells, and Merkel cells. Of these, keratinocytes are the major component, constituting roughly 95 percent of the epidermis. This stratified squamous epithelium is maintained by cell division within the stratum basale, in which differentiating cells slowly displace outwards through the stratum spinosum to the stratum corneum, where cells are continually shed from the surface. In normal skin, the rate of production equals the rate of loss; about two weeks are needed for a cell to migrate from the basal cell layer to the top of the granular cell layer, and an additional two weeks to cross the stratum corneum.

The dermis is the layer of skin between the epidermis and subcutaneous tissue, and comprises two sections, the papillary dermis and the reticular dermis. The superficial papillary dermis with the overlying rete ridges of the epidermis, between which the two layers interact through the basement membrane zone. Structural components of the dermis are collagen, elastic fibers, and ground substance. Within these components are the pilosebaceous units, arrector pili muscles, and the eccrine and apocrine glands. The dermis contains two vascular networks that run parallel to the skin surface—one superficial and one deep plexus—which are connected by vertical communicating vessels. The function of blood vessels within the dermis is fourfold: to supply nutrition, to regulate temperature, to modulate inflammation, and to participate in wound healing.

The subcutaneous tissue is a layer of fat between the dermis and underlying fascia. This tissue may be further divided into two components, the actual fatty layer, or panniculus adiposus, and a deeper vestigial layer of muscle, the panniculus carnosus. The main cellular component of this tissue is the adipocyte, or fat cell. The structure of this tissue is composed of septal (i.e. linear strands) and lobular compartments, which differ in microscopic appearance. Functionally, the subcutaneous fat insulates the body, absorbs trauma, and serves as a reserve energy source.

Conditions of the human integumentary system constitute a broad spectrum of diseases, also known as dermatoses, as well as many nonpathologic states (like, in certain circumstances, melanonychia and racquet nails). While only a small number of skin diseases account for most visits to the physician, thousands of skin conditions have been described. Classification of these conditions often presents many nosological challenges, since underlying etiologies and pathogenetics are often not known. Therefore, most current textbooks present a classification based on location (for example, conditions of the mucous membrane), morphology (chronic blistering conditions), etiology (skin conditions resulting from physical factors), and so on. Clinically, the diagnosis of any particular skin condition is made by gathering pertinent information regarding the presenting skin lesion(s), including the location (such as arms, head, legs), symptoms (pruritus, pain), duration (acute or chronic), arrangement (solitary, generalized, annular, linear), morphology (macules, papules, vesicles), and color (red, blue, brown, black, white, yellow). Diagnosis of many conditions often also requires a skin biopsy which yields histologic information that can be correlated with the clinical presentation and any laboratory data.

Some or all of the following may be seen in someone with Gorlin syndrome:

1. Multiple basal-cell carcinomas of the skin

2. Keratocystic odontogenic tumor: Seen in 75% of patients and is the most common finding. There are usually multiple lesions found in the mandible. They occur at a young age (19 yrs average).

3. Rib and vertebrae anomalies

4. Intracranial calcification

5. Skeletal abnormalities: bifid ribs, kyphoscoliosis, early calcification of falx cerebri (diagnosed with AP radiograph)

6. Distinct faces: frontal and temporoparietal bossing, hypertelorism, and mandibular prognathism

7. Bilateral ovarian fibromas

8. 10% develop cardiac fibromas