Despite claims of rarity, chronic ulcerative colitis is being encountered with increasing frequency in developing countries. We report our experience with 112 patients with chronic ulcerative colitis diagnosed during a 10-year period between 1973 and 1982 in Tehran, Iran, and compare the demographic and clinical features of chronic ulcerative colitis on our patients with those reported from other countries. Significant differences included the mild course of the disease, the absence of skin manifestations, and the rarity of colorectal cancer in our patients. The importance of changing socioeconomic status and cultural factors on the development of chronic ulcerative colitis is briefly discussed. The extreme rarity of Crohn's disease in Iran is noted.