The purpose of the study was to classify psoriasis vulgaris basing on HLA phenotype and the age of onset of the disease. One hundred fifteen psoriatic patients were included into the study and divided into two groups: group I--with early onset (< 40 years) and group II--with late onset (> or = 40 years). Each group was subclassified according to Cw6 antigen expression: IA-Cw(6+)--early onset, IB-Cw(6-)--early onset, IIA-Cw(6+)--late onset, IIB-Cw(6-)--late onset. HLA class I antigen typing was performed in each of 115 subjects using the microlymphocytotoxicity test. HLA class II antigen typing was also performed in 20 randomly selected patients by means of the two-step microlymphocytotoxicity test. The occurrence Cw6, B13, B17 antigens was significantly increased in psoriatic groups in comparison with a control population. No difference between the groups was found with respect to class II antigen frequency. Cw6, B13 and B17 were the most specific markers for psoriasis with the early onset, whereas B27, Cw2, B44 and Cw5 seemed to be associated with the late type of the disease.