HL-A27 were found in 31 of 48 patients with Reiter's disease (65%) as compared with 8% of 2 103 health controls. It is suggested that HL-A27 is closely related to the inheritance of a special immune response leading to an increased susceptibility to a special type of reactive arthritis which Reiter's disease has in common with ankylosing spondylitis and a certain type of psoriatic arthritis.