There is substantial variability in the severity of features of A-T among affected individuals, and at different ages. The following symptoms or problems are either common or important features of A-T:

- Ataxia (difficulty with control of movement) that is apparent early but worsens in school to pre-teen years

- Oculomotor apraxia (difficulty with coordination of head and eye movement when shifting gaze from one place to the next)

- Involuntary movements

- Telangiectasia (dilated blood vessels) over the white (sclera) of the eyes, making them appear bloodshot. These are not apparent in infancy and may first appear at age 5–8 years. Telangiectasia may also appear on sun-exposed areas of skin.

- Problems with infections, especially of the ears, sinuses and lungs

- Increased incidence of cancer (primarily, but not exclusively, lymphomas and leukemias)

- Delayed onset or incomplete pubertal development, and very early menopause

- Slowed rate of growth (weight and/or height)

- Drooling particularly in young children when they are tired or concentrating on activities

- Dysarthria (slurred, slow, or distorted speech sounds)

- Diabetes in adolescence or later

- Premature changes in hair and skin

Many children are initially misdiagnosed as having ataxic cerebral palsy. The diagnosis of A-T may not be made until the preschool years when the neurologic symptoms of impaired gait, hand coordination, speech and eye movement appear or worsen, and the telangiectasia first appear. Because A-T is so rare, doctors may not be familiar with the symptoms, or methods of making a diagnosis. The late appearance of telangiectasia may be a barrier to the diagnosis. It may take some time before doctors consider A-T as a possibility because of the early stability of symptoms and signs.

The first indications of A-T usually occur during the toddler years. Children start walking at a normal age, but may not improve much from their initial wobbly gait. Sometimes they have problems standing or sitting still and tend to sway backward or from side to side. In primary school years, walking becomes more difficult, and children will use doorways and walls for support. Children with A-T often appear better when running or walking quickly in comparison to when they are walking slowly or standing in one place. Around the beginning of their second decade, children with typical forms of A-T start using a wheelchair for long distances. During school years, children may have increasing difficulty with reading because of impaired coordination of eye movement. At the same time, other problems with fine-motor functions (writing, coloring, and using utensils to eat), and with slurring of speech (dysarthria) may arise. Most of these neurologic problems stop progressing after the age of about 12 – 15 years, though involuntary movements may start at any age and may worsen over time. These extra movements can take many forms, including small jerks of the hands and feet that look like fidgeting (chorea), slower twisting movements of the upper body (athetosis), adoption of stiff and twisted postures (dystonia), occasional uncontrolled jerks (myoclonic jerks), and various rhythmic and non-rhythmic movements with attempts at coordinated action (tremors).

Even though OMA is not always associated with developmental issues, children with this condition often have hypotonia, decreased muscle tone, and show developmental delays. Some common delays are seen in speech, reading and motor development

Telangiectasias are widened blood vessels that can develop anywhere on the skin, mucous membranes, whites of the eyes, and even in the brain. Telangiectasias are associated with multiple systemic signs, the most serious of which are unusual sensitivity to ionizing radiation, excessive chromosomal breakage, and a deficiency in the immune system. Ataxia telangiectasia results from defects in the ataxia telangiectasia mutated gene, which can cause abnormal cell death in various places of the body, including brain areas related to coordinated movement of the eyes. Patients with ataxia telangiectasia have prolonged vertical and horizontal saccade latencies and hypometric saccades, and, although not all, some patients show head thrusts.