We undertook a study to determine the incidence of hypopyon, as well as the most common anterior uveitis entities with which hypopyon is associated. A total of 216 patients with anterior uveitis were studied. The uveitis was acute in 155. Of the 155 patients, 11 (7.1%) had hypopyon. Nine of the 11 patients with hypopyon were positive for HLA B27. Of these nine, two had Reiter's syndrome and one had ankylosing spondylitis; the other six had no confirmed systemic disease. Of the two patients with hypopyon who were HLA B27-negative, one had mixed connective-tissue vascular disease, and one had idiopathic anterior uveitis. Of the 155 patients with acute anterior uveitis, 62 were HLA B27-positive. Thus, the incidence of hypopyon uveitis among HLA B27-positive patients was 14.5% (nine of 62 patients), whereas the incidence among HLA B27-negative patients was only 2.2% (two of 93 patients). These results suggest that HLA B27-related anterior uveitis is the most common cause of hypopyon uveitis, and that most patients with anterior uveitis associated with hypopyon will test positive for HLA B27. Although these results reflect a referral population, they should be of benefit in the treatment of patients with anterior uveitis.