Utilising the population based data resources of the Rochester Epidemiology Project, we determined the incidence and prevalence of chronic ulcerative colitis among Rochester, Minnesota, residents over the 20 year period, 1960-79. One hundred and thirty eight cases met diagnostic and residency criteria, for an overall age, and sex adjusted chronic ulcerative colitis incidence rate of 15.0 per 100,000 person years. The male:female ratio of age adjusted rates was 1.5:1. Age specific incidence was roughly bimodal in appearance but was not consistent in different patient subgroups. On 1-1-80, there were 120 Rochester residents with a history of chronic ulcerative colitis, corresponding to a prevalence rate of 225.2 per 100,000 population. Fifty three per cent of chronic ulcerative colitis incidence cases were 'definite' and 47% were 'probable', the former requiring consistent observations for at least six months. The definite group had proportionately more men and disease of greater extent and severity. Pancolitis comprised about one-third of all cases (4.6 per 100,000 person years). Proctitis and distal disease (7.1 and 2.0 per 100,000 person years) made up most of the rest. One-fourth of all patients had 'severe' or 'moderate' disease (3.8 per 100,000 person years), while the remainder had either 'mild' or 'transient' chronic ulcerative colitis (11.2 per 100,000 person years). In residents of Rochester, Minnesota, chronic ulcerative colitis is most often a mild disease. The over-representation of severe or complicated examples that results from selected referral to major centres probably distorts the natural clinical spectrum of the disease.