HLA antigen frequencies were studied in 158 patients with psoriatic arthritis, and compared to those of 101 patients with uncomplicated psoriasis and 243 healthy controls. The HLA antigens B16, B17, B27, B39 and Cw6 were associated with psoriatic arthritis. No associations between DR antigens and psoriatic arthritis were demonstrated. However, the subset of patients with rheumatoid-like arthritis demonstrated an increase in DR4. Uncomplicated psoriasis patients had higher frequencies of B17, Cw6 and DR7 than patients with psoriatic arthritis, while B7 and B27 correlated with development of arthritis. HLA-B27, Cw2 and DRw52 were associated with back involvement, whereas B38 and B39 were associated with polyarthritis. HLA-B7, B13 and DR7 correlated with milder disease in patients with psoriatic arthritis.