In order to examine the relationship between cigarette smoking and ulcerative colitis (UC), we carried out a case-control study of smoking habits in 84 patients with UC and in two different types of controls: 84 taken from a hospital population and 84 from the general population, matched to the cases for age, sex, school education and social class. Non-smokers were found to have a greater UC risk than smokers in both internal controls (Odds ratio = 4.6, 95% CL = 1.8-11.7) and external controls (OR = 10, 95% CL = 3.6-26.9). There was a larger number of ex-smokers among patients with UC than among the controls. Evaluations at the time of disease onset also demonstrated that ex-smokers had a greater relative risk. Furthermore, 84% of the ex-smoker patients had stopped smoking before the onset of symptoms. Conditional multiple logistic regression with the introduction of alcohol (wine) and coffee as additional variable factors to age, sex, education and social class confirmed the independent protective effect of smoking on UC (OR for internal controls = 4.2, 95% CL = 1.7-10.2, OR for external controls = 5.7, 95% CL = 2.3-14.1). These results indicate that non-smokers and especially ex-smokers of cigarettes have a greater risk of UC, and thus confirm the results of other studies.