Bronchial responsiveness was studied by histamine challenge in 423 school children with mean (SEM) age of 10.85 (0.05) years living in two districts of Hong Kong with contrasting levels of air pollution. Differences between districts of residence were observed, with a higher prevalence of bronchial hyperreactivity (BHR) in children living in the more polluted district (chi 2 = 7.74, df = 3, p = 0.052). Bronchial hyperreactivity was defined as a 20 percent or greater drop in FEV1. The district effect remained after exclusion of children with a history of wheeze and those diagnosed asthmatic for prevalence of BHR (chi 2 = 8.93, df = 3, p = 0.030) and for degree of bronchial reactivity (BR) after adjustment for other socioeconomic factors and smoking in the home (z = 1.97, p = 0.049). Bronchial reactivity was defined as the percentage drop in FEV1 per cumulative histamine dose from postsaline to end dose. The results demonstrate that studies on bronchial responsiveness can be used to assess the effects of air quality on the respiratory health of children and will be employed to measure the impact of new air quality control measures in Hong Kong.