From 5 centres 154 patients with ulcerative colitis and a history of smoking completed a questionnaire about the temporal relationship between diagnosis of colitis and cessation of smoking. One hundred and thirty-eight of these were ex-smokers and 107 (69.5%) had stopped smoking before the diagnosis of colitis was made. Fifty-six patients (52%) developed colitis within 3 years of stopping smoking. The findings suggest that smoking exerts protective effect against the development of ulcerative colitis.