The frequency of associated rheumatic diseases was studied in 271 patients with acute anterior uveitis (AAU). In a retrospective examination of 154 patients with AUU (mean follow-up period of 6 years) associated rheumatic symptoms were observed in 64 (41.6%). Forty-one patients (26.6%) had ankylosing spondylitis and 39 (25.3%) manifestations of Reiter's disease. Radiographic sacro-iliitis was seen in 35 (34%) of 103 consecutive x-ray examined patients with AAU. Furthermore, in another series of 38 patients, who all, in addition to having AAU, also complained of low back pain or had manifestations of Reiter's disease, 23 (60.5%) had radiographic sacro-iliitis. Classical ankylosing spondylitis was more frequent in men with AAU whereas milder forms of the disease occurred more equally in both sexes. HLA-B27 occurred in 35 (87.5%) of 40 HLA-typed patients with AAU. Associated rheumatic diseases occurred in 18 (51.4%) of the 35 HLA-B27 positive patients but in none of the HLA-B27 negative patients. The results support the hypothesis that a pleiotropic HLA-B27 associated gene may determine the susceptibility to AAU, sacro-iliitis, ankylosing spondylitis, and Reiter's disease.