The role of HL-A antigens in determining predisposition and clinical expression of variant forms of spondylitis was examined in 52 patients. HL-A 27, an antigen significantly correlated with ankylosing spondylitis, was found in 19 (63 per cent) of 30 patients with Reiter's syndrome. In 14 patients with psoriatic arthritis, this antigen was present only in the six patients who had spondylitis. The presence of HL-A 27 in Reiter's syndrome was correlated with the presence of uveitis, severe constitutional manifestations, and a chronic or relapsing course. The presence of HL-A 27 is associated with a genetic predisposition to Reiter's syndrome and with certain similarities in clinical expression of the variant forms of spondylitis.