To assess the changing distribution of disease within the large bowel in patients presenting with ulcerative proctitis or proctosigmoiditis. To evaluate the influence of clinical exacerbations, smoking, parity and family history in disease extension. Retrospective single-centre study in a university hospital. Case records of patients presenting over a 40-year period were examined to evaluate the clinical course and disease distribution from initial presentation to final follow-up. For each patient whose disease extended to the more proximal colon, an age- and sex-matched control patient was identified whose disease remained confined to the original site. Patients completed a questionnaire to provide information on family history, smoking and parity. The differences in clinical exacerbations, family history, smoking and parity were then compared between the two groups. Among 145 patients presenting with proctitis or proctosigmoiditis followed prospectively for a median period of 10.9 years, the disease extended in 53 patients. Using actuarial methods the disease progressed beyond the rectosigmoid area in 16% of patients at 5 years and 31% at 10 years. Among the patients whose disease progressed, progression was preceded in 68% of cases by a clinical exacerbation of the colitis. When patients whose disease progressed were compared with those whose disease remained confined to the original site, no differences were detected in the number of clinical exacerbations, smoking habit, family history or parity. The factors that are associated with the extension of colitis are probably different from those that predisposed an individual to develop inflammatory bowel disease initially.