The effects of smoking on the localisation and clinical course of Crohn's disease is evaluated in 231 patients. Heavy smokers (greater than 10 cigarettes/day) had an increased risk of operation at least once--odds ratios for heavy smokers compared with never smokers after five and 10 years were 1.14 and 1.24 respectively (p = 0.03 and p = 0.017). The risk of further operations was even higher and after 10 years the odds ratio was 1.79 (p = 0.015). The accumulated number of fistulae and/or abscesses was higher for smokers than for never smokers (p = 0.046). Patients with a high life time tobacco exposure (greater than 150 cigarette years) and heavy smokers (greater than 10 cigarettes/day) had small bowel disease more often than patients with lower life time exposure (less than or equal to 150 cigarette years) and patients smoking less than or equal to 10 cigarettes/day (p = 0.002 and p = 0.045 respectively). The course of Crohn's disease analysed in different ways was unfavourable for smokers, especially heavy smokers. Patients with Crohn's disease should be dissuaded from smoking.