Histocompatibility antigen frequencies were studied in a group of 100 patients with psoriatic arthritis (PSA) and were compared to a group of 80 patients whose psoriasis was restricted to skin lesions (PSC). The antigens B13, BW57 (17), CW6 were significantly increased in PSC while BW57 (17), BW39, CW6 and CW7 were increased in PSA. No DR or MT antigen was elevated in frequency when compared to normal controls. The significant information which results from this study includes: 1) a failure to confirm previous reports on HLA-DR antigen increased frequencies; 2) an association, in this population, of BW39 with PSA and not BW38; 3) finding of a closer link of PSC and PSA with HLA-C rather than HLA-B antigens; 4) an estimate of relative risk for patients with PSC to develop PSA.