The prevalence of allergic rhinitis, a common respiratory disorder, may be rapidly increasing. Epidemiological studies, however, indicate little about its association with climatic factors and air pollution. The relationship between traffic-related air pollutants and allergic rhinitis in middle-school students was therefore investigated.In a nationwide survey of middle-school students in Taiwan conducted in 1995/1996, the lifetime prevalence of physician-diagnosed allergic rhinitis and typical symptoms of allergic rhinitis were compared with air-monitoring station data on temperature, relative humidity, sulphur dioxide (SO2), nitrogen oxides (NOx), ozone (O3), carbon monoxide (CO) and particulate matter with a 50% cut-off aerodynamic diameter of 10 µm (PM10).A total of 331,686 nonsmoking children attended schools located within 2 km of 55 stations. Mean (range) annual exposures were: CO 853 (381–1,610) parts per billion (ppb), NOx 35.1 (10.2–72.4) ppb, SO27.57 (0.88–21.2) ppb, PM1069.2 (40.1–116.2) µg·m−3, O321.3 (12.4–34.1) ppb, temperature 22.9 (19.6–25.1)°C, and relative humidity 76.2 (64.8–86.2)%. The prevalence of physician-diagnosed allergic rhinitis was 28.6 and 19.5% in males and females, respectively, with prevalence of questionnaire-determined allergic rhinitis 42.4 and 34.0%. After adjustment for age, parental education and history of atopic eczema, physician-diagnosed allergic rhinitis was found to be associated with higher nonsummer (September–May) warmth and traffic-related air pollutants, including CO, NOx and O3. Questionnaire-determined allergic rhinitis correlated only with traffic-related air pollutants.Nonsummer warmth and traffic-related air pollution, probably mediated through exposure to common allergens such as dust mites, are possible risk factors for allergic rhinitis in middle-school-aged children.