Previous studies have reported relationships between adverse respiratory health outcomes and residential proximity to traffic. Diesel emissions, a major source of elemental carbon (EC) soot in urban areas, are suspected as being causal in these associations. Daily 24-hr personal samples of fine Particulate Matter air pollution (PM_2.5), including the EC fraction, were collected for forty fifth-grade children (10-12 years old) with asthma at four South Bronx schools (10 children per school) during approximately one month each. Spirometry (via portable AM1) and symptom scores were recorded several times daily. Higher personal EC exposures were found for subjects living close to a highway (< 550 ft (168 m)), and a significant linear relationship of home distance from a highway was found with personal EC concentrations (up to 1000 ft. (304 m)). Health effects results indicated the EC traffic-related fraction of the PM_2.5 as the one most consistently associated with symptoms and lung function outcomes. Significantly elevated same-day relative risks of cough, wheeze, shortness of breath, and total symptoms, and a nearly significant decline (0.05 > p < 0.10) in Peak Expiratory Flow (PEF) was found with an increase in personal EC, but not personal PM_2.5 mass. Furthermore, significant relative risks of symptoms were larger with lagged vs. same day EC concentrations for both personal and school-site measurements, suggesting possible delayed/cumulative effects. Associations found with PM _2.5 mass, ozone, and indicators of non-traffic PM (i.e., PM_2.5 sulfur and fine PM minus estimated diesel particulate matter (DPM) (EC/diesel factor)) did not confound the significant EC-health associations. The EC effect estimate was also robust to addition of traffic-related gaseous pollutants (NO₂ and SO₂). These results support the assumption by previous epidemiological studies that distance from a highway is a useful index of personal traffic pollution exposure, and suggest that the diesel fraction of PM_2.5 is most responsible for pollution-related asthma exacerbations among children living proximal to roadways.