Histocompatibility (HL-A) antigen typing was performed on 82 patients with psoriatic arthritis. The prevalance of HL-AB27 was increased (28%) compared with that in a control group (7-5%), and this was most marked in those with spinal changes. The prevalence in patients with sacroiliitis and syndesmophytes was 75%, with sacroiliitis alone 78%, but with syndesmophytes and normal sacroiliac joints only 36%. The overall prevalence in psoriatic spondylitis was 60%, with a prevalence of 71% in those fulfilling the New York criteria for anklosing spondylitis. The prevalence was not increased in those with only peripheral arthritis (11%). The prevalence of HLA-B13 was decreased in those with psoriatic spondylitis, though this was not statistically significant. The overall prevalence of HLA-BW17 was increased, this being particularly so in those with syndesmophytes, but values were not significant.