Ulcerative colitis is rare in black populations of sub-Saharan African. Only 18 cases were reported up to 1975. Since then, in four years, an additional 13 patients have been diagnosed at Baragwanath Hospital, Johannesburg. This hospital, with 2,700 beds, serves a population of approximately 1 1/2 million blacks. The low frequency of ulcerative colitis is in accord with the uncommonness of other noninfective large bowel diseases and contrasts with their high prevalences in white populations. Reasons for the uncommonness of ulcerative colitis in blacks are not known but consumption of their still largely traditional diet, insufficient exposure to environmental changes linked with urbanization and genetic factors may be responsible. All patients in this series, save one, were females. They were urbanized, belonged to upper social and educational strata and consumed a Western, or partially Westernized diet.