Histocompatibility (HLA) testing was performed on a total of 171 black Americans, 129 controls, and 42 patients with iridocyclitis. None of the patients had a history of rheumatological disease. The phenotype frequency of HLA B8 was greatly increased among patients than the control group (exact p = 0.0010). Most striking was the almost identical phenotypic frequency of B27 found in patients and in the control group (exact p = 0.07087). No HLA DR antigen in the patient group was found to be significantly raised when compared with controls. The HLA B8 associated diseases was seen more in females, was bilateral and nongranulomatous, had no systemic disease associated with it, and left patients with visual handicap. The HLA B8 antigen has been associated with various entities of presumed autoimmune origin, and these data suggest that iridocyclitis, at least in blacks, may be of similar aetiology.