The histocompatibility antigen HLA-B27 was identified in 12 out of 33 patients with acute non-granulomatous anterior uveitis. This is a frequency of 36.36%, compared with 4.72% in controls. Seven patients had in addition evidence of systemic disease, including ankylosing spondylitis, sacroiliitis, Reiter's disease, Still's disease, and rheumatoid arthritis. Five of these were HLA-B27 positive, which suggests that the uveitis in many of these cases has a similar aetiology to the uveitis in those with rheumatic disease. It appears that the more severe cases of acute anterior uveitis are related more frequently to the presence of HLA-B27.