We conducted a case-control study to assess the role of exposure to environmental tobacco smoke (ETS) in the development of wheezing bronchitis in children. The study included 199 children age 4 months to 4 years, who were treated in hospital for wheezing bronchitis, and 351 population controls of the same age group. We estimated exposure to ETS from urinary cotinine measurements as well as from questionnaires to parents. The median urinary cotinine concentration was 5.7 micrograms per liter in cases and 4.4 micrograms per liter in controls. Breast-feeding was related to urinary cotinine excretion in children with smoking mothers. The risk of wheezing bronchitis increased in relation to parental smoking and urinary cotinine concentration. This effect was most prominent in children up to 18 months of age, among whom the relative risk was 3.3 in those with a urinary cotinine level exceeding 20 micrograms per liter (95% confidence interval = 1.5-7.6). Our data confirm that ETS is an important risk factor for wheezing bronchitis in children and indicate that a single urinary cotinine measurement offers no major advantages to questionnaire data for assessment of long-term exposure to ETS.