: Cigarette smoking is associated with a more severe course of Crohn’s disease, but individual factors determining this effect are poorly known and it is not clear whether smoking cessation is associated with an improvement in the disease activity.


: To assess the factors determining the harmful effect of smoking in individuals with Crohn’s disease.


: A total of 622 consecutive patients with Crohn’s disease and Crohn’s disease activity index <200 were enrolled in a prospective 12–18 month cohort study. Patients were classified as current smokers, former smokers, or non-smokers. Alcohol consumption, oral contraceptive use, body mass index, and blood lipid levels were also recorded. The main outcome measure was the rate of flare-up.


: A total of 139 current smokers (46%) developed a flare-up, vs. 79 non-smokers (30%) and 13 former smokers (23%). The relative risk of flare-up adjusted for confounding factors was 1.35 (1.03–1.76) in current smokers. This risk was increased in patients with previously inactive disease and in those who had no colonic lesions. It became significant above a threshold of 15 cigarettes per day. Former smokers behaved like non-smokers. Obesity, dyslipidaemia, and alcohol consumption had no significant effect.


: Current smoking, particularly heavy smoking, markedly increases the risk of flare-up in Crohn’s disease. Former smokers have a risk similar to that of non-smokers.