Background: The association between ambient temperature and lung function in children with asthma is still uncertain.Methods: A panel of 270 children (7-12 years) with asthma was recruited from six Australian cities. They performed three successive forced expiratory maneuvers twice daily for four weeks. The highest peak expiratory flow rate (PEF) and forced expiratory volume in one second (FEV1) were stored for each session. Daily data were obtained on ambient temperature, relative humidity and air pollution, during the same period. Mixed models were used to examine the effects of temperature on lung function, controlling for individual characteristics and environmental factors.Results: Ambient temperature was negatively related to both morning and evening PEF and FEV1 for lag 0-3 days. In general, the effects of temperature were stronger in boys than in girls for evening PEF, while the effects were stronger in girls for evening FEV1. Children with asthma living in southern cities were more sensitive to high temperature than those in the northern most city.Conclusions: Higher ambient temperature is associated with lower lung function in children with asthma. Preventive health policies will be required for protecting children with asthma from the increasing frequency of high temperatures.