Histocompatibility antigens (HLA-A, B) have been studied in 70 patients with ulcerative colitis of Greek origin. An increased frequency of HLA-A11, B7 and B13 (RR 2.1, 2.0 and 4.5, respectively) and a decreased frequency of HLA-B14 (RR 0.2) were found. Severe course of the disease was associated mainly with HLA-B13 (RR 5.5) whereas extensive spread of the colitis was associated with HLA-A11 and B7 (RR 2.6 and 3.2). Late onset of the disease (greater than 30 years) was characterized by increased frequency of HLA-A11, B7 and B13.