Adult cigarette smoking is associated with the development of Crohn's disease and protection from the development of ulcerative colitis. Children usually are nonsmokers whose risk of developing inflammatory bowel disease (IBD) may be related to passive smoking. The purpose of this matched case-control study was to evaluate passive smoking exposure in 72 nonsmoking children with recently diagnosed IBD (39 with ulcerative colitis and 33 with Crohn's disease), and in an equal number of peer-nominated controls. Passive smoking exposure at birth was significantly associated with the development of IBD (odds ratio 3.02, 95% confidence interval 1.28-7.06). The effect was greater in Crohn's disease (odds ratio 5.32) than in ulcerative colitis (odds ratio 2.19). Maternal smoking at birth also was significantly associated with the development of IBD (odds ratio 2.09, 95% confidence interval 1.02-4.29), an effect that also was greater in Crohn's disease than in ulcerative colitis. There was a dose-response relationship between packs smoked per day and IBD, and packs smoked at home per day and IBD. At symptom onset, the risk of developing IBD from passive smoking exposure was increased but was not significant (odds ratio 1.88, 95% confidence interval 0.84-4.18). The magnitude of the effect was greater in Crohn's disease than in ulcerative colitis, and the association demonstrated dose-response. In conclusion, passive smoking exposure and maternal smoking at birth and, to a lesser extent, passive smoking exposure at symptom onset are associated with an increased risk of developing IBD in children. The association is stronger in Crohn's disease than in ulcerative colitis, and there is a dose-response effect. The specific toxic exposure is more likely to be inhaled rather than passed through the placenta or in breast milk.