The frequencies of HLA antigens were studied in 101 Italian patients with psoriatic arthritis. The total group showed a significant increase in frequency of A1 and B38, and a reduction of B5 when compared to healthy controls. No association between DR and/or DQw antigens and PA were demonstrated. The comparisons between the clinical subgroups and normal controls revealed a significant association of B38 with asymmetric peripheral arthritis, B27 and B39 with spondylitis (with or without peripheral involvement). When intergroup comparison were made, the patients with spondylitis had an increase in frequency of B27 and DQw3 as compared to those with symmetric and asymmetric peripheral disease. DR4 and DRw53 were associated with earlier age of onset of arthritis. There were also significant associations between DQw3 and severe disease, and between A9, B5 and presence of erosions and joint space narrowing. No association with DR4 was showed in a subgroup of patients with symmetric polyarthritis without DIP involvement.