Details of the smoking history were obtained from 288 patients with ulcerative colitis who were more than 15 years of age when colitis developed. Men who had previously smoked presented with colitis at a later age than lifelong non-smokers (mean age difference 15.2 years). The proportion of ex-smokers in this group of patients with colitis was more than twice that expected in the general population. The interval between cessation of smoking and subsequent onset of colitis in ex-smokers was found to be relatively short and in two-thirds of patients occurred within seven years. A tentative model to explain the association between smoking and subsequent colitis is proposed. We suggest that whilst current smokers may have a lower incidence of colitis, ex-smokers appear to be at increased risk of developing the disease. An alternative explanation is that smoking may act in a manner analogous to sulphasalazine.