Sixty-six patients with psoriatic arthropathy were subdivided into nine groups on the basis of the presence of peripheral arthritis, axial disease whether or not fulfilling the New York criteria for ankylosing spondylitis both associated and not associated with peripheral arthropathy and bilateral or monolateral lateral sacro-iliitis. Only the group with axial disease sacroiliitis+spondylitis) without peripheral arthritis and not fulfilling the NY criteria showed a truly increased B27 prevalence. However, in this atypical group, only 2 patients had a true ankylosing pattern-like spondylitis. On the other hand, in the group with axial disease fulfilling the NY criteria, only one of 9 patients was B27+. We conclude that B27 is not a true marker of axial involvement in psoriatic arthropathy.