Few studies have investigated the relationship between vehicle exhaust and the new onset of asthma among adults. The aim of the present prospective cohort study was to investigate the relationship between the cumulative incidence of asthma and onset of asthma among adults and vehicle exhaust concentrations at home. Participants from three Swedish cities included in the Respiratory Health in Northern Europe cohort constituted the study population. Exposure at each participant's home was calculated using dispersion models. We also used <50 m distance to nearest major road as a more simple indicator of exposure. The adjusted model included 3,609 participants, of which 107 were classified as onset cases and 55 as true incident cases of asthma. There was a positive association between asthma onset (odds ratio (OR) per 10 microg x m(-3) 1.46, 95% confidence interval (CI) 1.07-1.99) and incident asthma (OR per 10 microg x m(-3) 1.54, 95% CI 1.00-2.36) and the levels of nitrogen dioxide (NO(2)), which remained statistically significant after adjusting for potential confounders. The relationship between asthma and NO(2) was not significantly modified by sex, hay fever or wheeze. The risk of developing asthma was also significantly related to living close to a major road. The current study suggests that elevated levels of vehicle exhaust outside the home increase the risk of onset and incident asthma among adults.