Previous studies have demonstrated an association between air pollution and asthma exacerbation. Less understood is the effect of elemental carbon (EC), and the interaction of EC with temperature, on increases in pediatric asthma emergency department visits and how these relationships change across the seasons in a metropolitan area with several industries and relatively low air pollution. Measurements of EC, ozone (O(3)), sulfur dioxide (SO(2)), and total oxides of nitrogen (NO(x)) were available from the St. Louis EPA Supersite for June 1, 2001 to May 31, 2003. We obtained ICD-9 information on 281,763 pediatric ED visits from 27 hospitals in the St. Louis, MO metropolitan area. The relationship between EC and pediatric asthma ED visits, controlling for season, weekend exposure, allergens, and other pollutants known to exacerbate asthma, was assessed using Poisson generalized estimating equations using a 1-day lag between exposure and ED visit. We evaluated the interaction of EC and temperature and EC and weekend vs. weekday exposure. An interaction effect existed between EC and temperature for 11-17-year-olds during the summer and winter seasons. During the summer, a 0.10 microg/m(3) increase in EC resulted in a 9.45% increase in asthma ED visits among 11-17-year-olds (95%CI = 1.02,1.17) at the median seasonal temperature (86.5 degrees F). This risk increased with increasing temperature. During the winter, a 0.10 microg/m(3) increase in EC resulted in 2.80% increase in asthma ED visits among 11-17-year-olds (95%CI = 1.01,1.05) at the median seasonal temperature (43.3 degrees F). This risk increased with decreasing temperature. Among 11-17-year-olds, daily number of asthma ED visits is associated with increased levels of EC at higher temperatures in the summer and lower temperatures in the winter.