One hundred and sixty-nine patients with acute anterior uveitis were studied for the presence of HLA-B27 tissue type, radiological evidence of ankylosing spondylitis, and a history of back pain. 60% were male; 45% were HLA-B27+. The male:female ratio in the HLA-B27+ group was the same as in the whole group. 24% had radiological evidence of ankylosing spondylitis, and, of these, 83% were HLA-B27+ while 17% were HLA-B27-. There was a definite correlation between the severity of the ankylosing spondylitis and preponderance of males in the HLA-B27+ group of patients. Significant back pain occurred in 60% of HLA-B27+ patients but in only 14% of HLA-B27- patients. The importance of the selection of patients and of consistency in the use of diagnostic criteria in studies relating to ankylosing spondylitis and acute anterior uveitis is stressed.