HLA antigens were studied in 60 patients with ulcerative colitis, 10 with Crohn's disease and 120 healthy Japanese controls. HLA typing was performed by the microdroplet lymphocyte cytotoxicity test.B5 was recognized in 62% of the patients with ulcerative colitis, whereas in 40% of controls (p smaller than 0.01). Furthermore, in these patients B7 was decreased in incidence (p smaller than 0.05) and there was the highest relative risk value for A3 (R.R = 6.3). When the patients were classified into 2 groups according to the extent of the lesions, left-sided or total colitis group and proctitis group, B5 was recognized in 65% of the patients with left-sided colitis or total colitis; and this increase was statistically significant. However, no significant difference in the incidence of B5 was recognized between the patients with proctitis and controls, probably because the number of patients studied was too small for evaluation. In the patients with Crohn's disease, B27 was recognized in one patient, showing an increase in incidence (p smaller than 0.001), although the number of patients studied was very small.