The acute respiratory effects of ambient air pollution were studied in a panel of 73 children with chronic respiratory symptoms in the winter of 1990 to 1991. The participating children were selected from all children aged 6 to 12 yr in Wageningen and Bennekom, two small, nonindustrial towns in the east of the Netherlands. Peak flow was measured twice daily with MiniWright meters. A diary was used to register the occurrence of acute respiratory symptoms and medication use by the children. Exposure to air pollution was characterized by the ambient concentrations of sulfur dioxide (SO2), nitrogen dioxide (NO2), black smoke (BS), and particulate matter less than 10 microns (PM10). Associations between air pollution concentrations and health outcomes were analyzed using time series analysis. During the study period an air pollution episode occurred, with moderately elevated concentrations of PM10 and SO2. There were 6 days with 24-h average PM10 concentrations in excess of the WHO suggested lowest observed effect level of 110 micrograms/m3. After adjustment for ambient temperature, there were small but statistically significant negative associations of PM10, BS, and SO2 with both morning and evening PEF. There was a consistent positive association between PM10, BS, and SO2 with the prevalence of wheeze and bronchodilator use. Overall, the observed associations suggest a mild to moderate response to these moderately elevated levels of air pollution in a group of potentially sensitive children.