Of all cancers involving the same class of blood cell, 2% of cases are cutaneous T cell lymphomas. CTCL is more common in men and in African-American people. The incidence of CTCL in men is 1.6 times higher than in women.

There is some evidence of a relationship with human T-lymphotropic virus (HTLV) with the adult T-cell leukemia/lymphoma subtype. No definitive link between any viral infection or environmental factor has been definitely shown with other CTCL subtypes.

The presentation depends if it is mycosis fungoides or Sézary syndrome the most common, though not the only types.

Among the symptoms for the aforementioned types are:lymphadenopathy, hepatosplenomegaly, and non-specific dermatitis.

It is rare for the disease to appear before age 20, and it appears to be noticeably more common in males than females, especially over the age of 50, where the incidence of the disease (the risk per person in the population) does increase. The average age of onset is between 45 and 55 years of age for patients with patch and plaque disease only, but is over 60 for patients who present with tumours, erythroderma (red skin) or a leukemic form (the Sézary syndrome). The incidence of mycosis fungoides was seen to be increasing till the year 2000 in the United States, thought to be due to improvements in diagnostics. However, the reported incidence of the disease has since then remained constant, suggesting another unknown reason for the jump seen before 2000.

The cause of mycosis fungoides is unknown, but it is not believed to be hereditary or genetic in the vast majority of cases. One incident has been reported of a possible genetic link. It is not contagious.

The disease is an unusual expression of CD4 T cells, a part of the immune system. These T cells are skin-associated, meaning that they biochemically and biologically are most related to the skin, in a dynamic manner. Mycosis fungoides is the most common type of cutaneous T-cell lymphoma (CTCL), but there are many other types of CTCL that have nothing to do with mycosis fungoides and these disorders are treated differently.