To determine the hereditary pattern of acute anterior uveitis, we examined seven families-one family with four cases, one family with three cases, and four families with two cases of acute anterior uveitis. A total of 16 patients with uveitis and 28 relatives underwent ocular examinations and serologic determination of HLA antigens, protease inhibitor alpha 1-antitrypsin typing, and X-ray examination of sacroiliac joints. Sacroiliitis was found in 11 of 16 patients (68.8%) and incomplete Reiter's disease in seven of 16 (43.8%). HLA-B27 antigen was found in all patients with uveitis and in six of 21 (28.6%) relatives. HLA-Cw1 antigen was found in nine of 14 (64.3%) patients with uveitis, often in the same haplotype with HLA-B27 antigen. HLA-DR4 antigen was found in seven of 11 (63.6%) patients with uveitis. There was no significant association of the alpha 1-antitrypsin variant MZ with uveitis. We believe that a pleiotropic gene associated with HLA-B27 antigen with autosomal dominant inheritance, incomplete penetrance, and variable expressivity may determine susceptibility to acute anterior uveitis in linkage disequilibrium with HLA-Cwl antigen.