We studied the association between air pollution and cardiopulmonary fitness among 821 schoolchildren aged 8 to 12 from two districts with different air quality in Hong Kong. Their parents completed a respiratory questionnaire, and the maximum oxygen uptake (VO2max) of the children was assessed using the multistage fitness test. After adjusting for potential confounding factors, children in the high pollution district had a significantly lower VO2max than those in the low pollution district (27.9 mL·kg-1·min-1 vs. 29.8 mL·kg-1·min-1). Habitual physical exercise was associated with a higher VO2max in the low-pollution district but not in the high-pollution district. Air pollution adversely affected the VO2max in children, and physical exercise in a polluted environment might not have beneficial effect on cardiopulmonary fitness.