We studied the clinical features and prognosis of 73 patients with HLA-B27 positive and 71 patients with HLA-B27 negative acute anterior uveitis using computer analysis of more than 50 variables per patient. The patients with HLA-B27 positive acute anterior uveitis showed the following characteristics which were significantly different from patients with HLA-B27 negative acute anterior uveitis: younger age at onset; high male to female ratio; frequent unilateral alternating eye involvement; severe ocular symptoms during activity, such as presence of fibrin in the anterior chamber; absence of mutton fat keratic precipitates; high incidence of ocular complications; and frequent association with seronegative spondyloarthropathies. Despite the difference of disease severity and incidence of complications, the long-term visual outcome did not differ significantly between the two groups. No distinctions were observed when patients with HLA-B27 positive acute anterior uveitis were subdivided according to sex or the presence of ankylosing spondylitis. HLA-B27 positive acute anterior uveitis formed a distinct clinical entity associated with serious prognosis as compared to HLA-B27 negative acute anterior uveitis.