The proportion of smokers in this study was significantly lower among patients with ulcerative colitis (13%) than among patients with Crohn's disease (47%), the difference being significant for both sexes and for the age groups both below and above 40 years. The proportion of male ex-smokers among patients with ulcerative colitis (28%) was higher than among patients with Crohn's disease (8%), whereas the proportions of non-smokers differed less. Many of the patients with ulcerative colitis who had a late onset were male ex-smokers. The smoking patients with ulcerative colitis were mainly women. They smoked less than the smoking patients with Crohn's disease and less than the ex-smokers in each group. No smoking patient with ulcerative colitis smoked greater than 20 cigarettes/day. In the group of male ex-smokers with ulcerative colitis, there was an accumulation of onsets during the 4 years after the definitive smoking stop. The number of colectomies performed on patients with ulcerative colitis did not vary with smoking habits. In the group of ex-smokers the colectomy was performed after the smoking stop in 19 out of 20 patients.