The smoking habits of 145 patients with irritable bowel syndrome (IBS) were compared with those of 63 patients with ulcerative colitis and 25 patients with Crohn's disease. Patients with IBS and ulcerative colitis smoked significantly less than those with Crohn's disease. There was no significant difference in the smoking prevalence between ulcerative colitis and IBS patients. There were significantly more ex-smokers in the ulcerative colitis group and two-thirds of these patients developed their colitis within a year of stopping smoking. It is suggested that any protective role postulated for smoking in the pathogenesis of ulcerative colitis should also be considered for IBS.