['Ambient particulate concentrations have been associated with variable physiologic effects in children with persistent asthma taking controller medications.', 'To determine whether exposure to particulate matter has immediate effects on asthma control in children with persistent disease.', 'In a school-based cohort, 73 children, primarily with moderate and severe asthma, were followed daily over one or two winters (2001-2002, 2002-2003) in Denver, Colorado. The association among ambient fine particulate, bronchodilator use, and urinary leukotriene E4 levels was assessed.', 'Daily concentrations of fine particulate peaked in the morning hours when children were commuting to school. In a multivariable analysis that controlled for meteorology, time trends, and upper respiratory infections, an increase of one interquartile range in morning maximum fine particulate levels was related to an average increase of 3.8% in bronchodilator usage at school (95% confidence interval [CI], 0.2-7.4; p = 0.04). Children with severe asthma demonstrated significantly stronger associations (8.1% increase; 95% CI, 2.9-13.4; p = 0.003) than those with mild/moderate disease (1.6% increase; 95% CI, -2.2-5.4; p = 0.41; p = 0.03 for difference between groups). Morning maximum fine particulate levels were also associated with urinary leukotriene E4 measured during school hours (average increase of 6.2% per interquartile range increase; 95% CI, 1.9-10.5; p = 0.006). These associations were not discernable when 24-h averaged concentrations were used.'] Are particulate levels associated with early asthma worsening in children with persistent disease?