We assessed the impact of summertime haze episodes on twice daily peak flow measurements of children in Philadelphia, Pennsylvania. Height-adjusted peak flows were regressed on weather and air pollution concentrations. Lower morning peak flows were associated with exposure to inhalable particulate matter (-2.94 liters/minute/18 microg/m3, 95% confidence limits (CL) = -0.56, -5.33), and fine sulfate particles (-2.44 liters/minute/8 microg/m3, 95% CL = -0.36, -4.51). Particle-strong acidity and the coarse particle mass were weakly associated with lower peak flow. Acutely lower peak flows in children were associated with fine sulfate particles, but only weakly with the acidity of the fine particles.