A case-control study was carried out in Stockholm, Sweden between 1984 and 1987 to evaluate the association of cigarette smoking and exposure to environmental tobacco smoke during childhood and the subsequent development of inflammatory bowel disease. Information on smoking was obtained by a postal questionnaire. The relative risk of Crohn's disease in current smokers compared with those who had never smoked was 1.33 (95% confidence limits 0.7; 2.6) in men and 4.99 (2.7; 9.2) in women; the corresponding results for ulcerative colitis were 0.96 (0.5; 1.8) and 0.72 (0.4; 1.4). The relative risk of ulcerative colitis in recent exsmokers compared with those who had never smoked was 2.18 (0.9; 5.0). Furthermore, an increase in the risk of Crohn's disease was found in those who were exposed to environmental tobacco smoke during childhood, the relative risk being 1.50 (1.0; 2.3). The corresponding relative risk of ulcerative colitis was 0.98 (0.6; 1.5).