OBJECTIVE: The purpose of this study was to investigate dose-response relationships between asthma symptoms and indoor nitrogen dioxide (NO2) and house dust mite allergen (HDM) in children. METHODS: Asthmatic children from 18 primary schools in Adelaide, Australia, kept a daily symptoms diary over 12 weeks. Home and classroom NO2 levels were measured repeatedly in winter 2000. HDM levels were obtained from beds. Lung function tests were performed at the beginning and at the end of the study period. RESULTS: Data on exposure and respiratory outcomes were gathered for 174 children. For school exposure, the estimated relative symptom rate (RR) for a 10-ppb increase in NO2 for difficulty breathing during the day was 1.09 (95% confidence interval [CI] = 1.03-1.15), at night 1.11 (95% CI = 1.05-1.18), and for chest tightness at night 1.12 (95% CI = 1.07-1.17). Significant symptom rate increases were also found for kitchen NO2 exposure. This was supported by a negative dose-response relationship between percentage predicted forced expiratory volume in 1 second and NO2 (-0.39%; 95% CI = -0.76 to -0.02) for kitchen exposure. Significant threshold effects using a 10-microg/g cutoff point for HDM exposure were established in the sensitized children for nighttime wheeze (RR = 3.62, 95% CI = 1.49-8.77), daytime cough (RR = 1.64, 95% CI = 1.14-2.36), and daytime asthma attack (RR = 1.95, 95% CI = 1.06-3.60). CONCLUSION: This study has established reliable risk estimates for exacerbations of asthma symptoms in children based on dose-response investigations of indoor NO2 and HDM.