The prevalence of HLA-A, B, C and DR antigens was determined and compared in 94 patients with reactive arthritis, 54 patients with ankylosing spondylitis (AS), 37 patients with inflammatory bowel disease (IBD) and in 1,010 apparently normal blood donors. The 185 patients all underwent ileocolonoscopy with biopsy of ileum, ileocecal valve and cecum. HLA-B27 was found elevated in the groups with reactive arthritis (48%, chi 2 = 82, p less than 0.0005) and the AS groups (78%, chi 2 = 157, p less than 0.0005), compared to healthy controls. HLA-Bw62 was significantly raised in the patients with reactive arthritis (34%, chi 2 = 73, p less than 0.0005) (particularly the HLA-B27 negatives (48%, chi 2 = 90, p less than 0.0005) and in the HLA-B27 negative patients with AS (25%, chi 2 = 5.5, p less than 0.02). This did not apply to the other patients with AS (4.7% NS). HLA-Bw62 could be associated with a specific clinical picture of asymmetrical pauciarticular arthritis, accompanied by enthesopathies and sacroiliitis classified as idiopathic reactive arthritis, especially when the disease is of enterogenic origin. The frequency of HLA-Bw62 was very high in patients with reactive arthritis and in patients with AS with active chronic (n = 39, 23%, chi 2 = 13, p less than 0.0005) and Crohn-like lesions (n = 14, 50%, chi 2 = 35, p less than 0.0005) on gut biopsy, normal in patients with acute lesions (n = 35, 11%, NS).(ABSTRACT TRUNCATED AT 250 WORDS)