We assessed the role of ventilation rate in homes in the development of bronchial obstruction during the first 2 years of life. We conducted a matched case-control study based on a cohort of 3,754 newborns in Oslo in 1992-93 that was followed for 2 years. The case series comprised 172 children with bronchial obstruction, and the control series was one-to-one matched for date of birth. Ventilation rate and other building characteristics were measured/collected in home visits, and questionnaires were used to obtain additional information. We found that the risk of bronchial obstruction was not directly associated with the ventilation rate in liters per second and per person (ventilation rate quartiles: 6.9, 11.5 and 17.6 l/s,p [liter/second and person]) in the homes (odds ratio; OR 0.98, 95% confidence interval (CI) 0.96-1.01). The odds ratios of bronchial obstruction were higher in the low air change group (air change rate < or =0.5 h(-1)) than in the high air change group owing to exposure to environmental tobacco smoke (low 1.8; 95% CI: 0.35-9.66; high 1.5; 95% CI: 0.74-3.20), dampness problems (low 9.6; 95% CI: 1.05-87.4; high 2.3; 95% CI: 0.83-6.39), and the presence of textile wall paper (low 3.7; 95% CI: 0.62-21.5; high 1.7; 95% CI 0.72-3.94) and plasticizer-containing surfaces (low 12.6; 95% CI: 1.00-159; high 2.6; 95% CI: 1.02-6.58). Our results are consistent with the hypothesis that low ventilation rates strengthen the effects of indoor air pollutants.