Traffic emissions are a major source of air pollution in Western industrialized countries. To investigate the association between traffic-related air pollution and parameters of atopy, we studied 317 children 9 years of age living near major roads in two urban areas and one suburban area of a city in West Germany. Atopic sensitization was analyzed by skin-prick testing and determination of allergen-specific serum immunoglobulin E. Parents recorded allergic symptoms in a symptom diary, and physicians assessed allergic diseases. Personal NO2 exposure and NO2 concentrations in front of each child's home were measured. Outdoor NO2 was a good predictor for traffic exposure but a poor predictor for NO2 exposure at the personal level. Atopy was found to be related to outdoor NO2 (odds ratio for the association between symptoms of allergic rhinitis and outdoor NO2 = 1.81; 95% confidence interval = 1.02-3.21) but not to personal NO2 (odds ratio for the association between symptoms of allergic rhinitis and personal NO2 = 0.99; 95% confidence interval = 0.55-1.79). When the analysis was restricted to urban areas, we found that hay fever, symptoms of allergic rhinitis, wheezing, sensitization against pollen, house dust mites or cats, and milk or eggs were associated with outdoor NO2. The results indicate that traffic-related air pollution leads to increased prevalence of atopic sensitizations, allergic symptoms, and diseases.