The frequencies of human lymphocyte antigens (HLAs) A, B, and Cw6 were studied in a group of 212 Greek patients with psoriasis and 202 control subjects. An increased frequency of B13, B16, and Cw6 antigens and a low frequency of B14 were noted in the group of all patients. However, there were striking differences in HLA phenotypes according to the age at onset of the disease. Early onset (<25 years of age) was associated with a high frequency of A1, B17, B37, and Cw6 antigens, whereas patients with late onset (>25 years of age) had a significantly lower incidence of Al, B17, and Cw6, which did not differ from that of the control subjects. Patients with onset of the disease >60 years of age had, in addition, an increased frequency of B16. Psoriatic erythroderma was characterized by an increase of Aw19 and a higher frequency of B13 than in the total group of patients with psoriasis. In patients with a familial incidence of the disease, there was no significant association with any particular HLA.