Although increases in inhalable particle (PM10) concentrations have been associated with acute reductions in the level of lung function and increased symptom reporting in children, including children with asthma, it is not clear whether these effects occur largely in asthmatic children, or even whether asthmatic children are more likely to experience these effects than children without asthma. To address these points, the following subgroups of children were selected from a survey population of all 2,200 elementary school children (6 to 13 yr of age) in a pulp mill community on the west coast of Vancouver Island: (1) all children with physician-diagnosed asthma (n = 75 participated), (2) all children with an exercise-induced fall in FEV1 without diagnosed asthma (n = 57), (3) all children with airway obstruction (FEV1/FVC < 0.76) without either of the above (n = 18), and (4) control children without any of the above (n = 56). The children were followed for as long as 18 mo with twice daily measurements of peak expiratory flow (PEF) and daily symptom diary recording. Maximum daily PM10 concentration was 159 microm/m3 (median, 22.1), but only 8 d (1.2%) had concentrations above 100 microg/m3. In an analysis that accounted for time-varying covariates, and serially correlated and missing data, for the entire sample of children, increases in PM10 were associated with reductions in PEF and increased reporting of cough, phlegm production, and sore throat. For the subgroup of children with diagnosed asthma, PEF in the time period with the highest PM10 concentrations fell by an estimated 0.55 L/min (95% CI, 0.06 to 1.05) for a 10 microg/m3 PM10 increase above the mean daily PM10 concentration of 27.3 microg/m3 and the odds of reported cough increased by 8% (95% CI, 0 to 16%); no consistent effects were observed in the other groups of children. It is concluded that children experience reductions in PEF and increased symptoms after increases in relatively low ambient PM10 concentrations, and that children with diagnosed asthma are more susceptible to these effects than are other children.