In the early summer of 1995, the acute respiratory effects of ambient air pollution were studied in a panel of 61 children, ages 7 to 13 yr, of whom 77% were taking asthma medication. Peak flow was measured twice daily with MiniWright meters at home and the occurrence of acute respiratory symptoms and medication use was registered daily by the parents in a diary. Exposure to air pollution was characterized by the ambient concentrations of ozone, PM10, and black smoke. During the study period, maximal 1-h ozone concentrations never exceeded 130 microg/m3, and 24-h black smoke and PM10 concentrations were never higher than 41 and 60 microg/m3 respectively. Associations of air pollution and health outcomes were evaluated using time series analysis. After adjusting for pollen, time trend, and day of the week, black smoke in particular was associated with acute respiratory symptoms and with medication use. Less strong associations were found for PM10 and ozone. These results suggest that in this panel of children, most of whom had asthma, relatively low levels of particulate matter and ozone in ambient air are able to increase symptoms and medication use.