The frequency distribution of HLA antigens was studied in a group of psoriatic patients with and without arthritis. HLA-BW38 was found significantly increased in the group with peripheral psoriatic arthritis. There were no differences in the clinical features of the arthritis in the BW38 positive (+) and BW38 negative (-) individuals. It is suggested that BW38 is a genetic marker and/or an indicator of peripheral joint involvement in psoriatic patients.