Ulcerative proctitis has by tradition been regarded as a subgroup of ulcerative colitis. Population-based epidemiological studies of ulcerative proctitis are, however, virtually nonexistent. In an epidemiological study of inflammatory bowel disease in the Uppsala Health Care Region, 1065 cases of ulcerative proctitis were diagnosed from 1965 through 1983. Males predominated, with the male to female ratio 1.4:1. Annual incidence rates were higher in urban than in rural areas. The annual incidence rates increased threefold from 2.8 per 10(5) to 6.6 per 10(5) during the period, affecting all age groups over 14 years of age, in both urban and rural areas and in both sexes. Differences in temporal trends and certain other epidemiological characteristics between ulcerative proctitis and extensive ulcerative colitis suggest that ulcerative proctitis is a specific disease whose etiology differs from that of extensive ulcerative colitis.