Many people, including people with asthma, experience short-term exposure to diesel exhaust (DE*) during daily activities. The health effects of such exposures, however, remain poorly understood. The present study utilized a real-world setting to examine whether short-term DE exposure would (1) worsen asthma symptoms, (2) augment airway inflammation, or (3) increase oxidative stress burdens. The study also examined exposure-response relations for several DE components and the contribution of background asthma severity to individuals' respiratory responses to DE exposure. Sixty people participated in the study; 31 had mild asthma and 29 had moderate asthma. Each participant completed an exposure and a control session. During the exposure session, participants walked for 2 hours along a heavily trafficked city street where motor vehicle access was restricted to buses and official taxicabs. These vehicles were powered by diesel engines. During the control session, participants walked for the same duration and at the same speed in a public park where motor vehicle traffic was prohibited. The concentrations of elemental carbon (EC), NO2, ultrafine particles (UFP), and particulate matter less than or equal to 2.5 microm in aerodynamic diameter (PM2.5) during exposure sessions were, on average, 4.8, 4.0, 3.4, and 2.0 times higher, respectively, than during control sessions. Increases in asthma symptom score and in the daily use of asthma reliever medication within the 7-day measurement period after exposure were not significant. Some effects on lung function were statistically significant. Compared with control sessions, forced expiratory volume in the first second (FEV1) was reduced 3.0% to 4.1%, and forced vital capacity (FVC) was reduced 2.8% to 3.7% in the 5 hours immediately after the exposure sessions. Analyses of biomarkers showed that the exposure sessions led to a significant reduction in exhaled breath condensate (EBC) pH and to significant increases in induced sputum neutrophils and myeloperoxidase (MPO). The changes in lung function indices (FEV1, FVC, and forced expiratory flow during the middle half of the FVC [FEF25-75]) were most consistently associated with UFP and EC exposures, whereas the changes in EBC pH were most consistently associated with NO2 exposure. In addition, NO2 had a significant effect on bronchial reactivity and on the amount of interleukin-8 (IL-8) in induced sputum; it also modified the UFP effect on EBC pH and the EC effect on exhaled nitric oxide (eNO). However, our findings cannot be taken as demonstrating a causal association with any measured pollutant, because the measured pollutant concentrations may simply represent the entire roadside diesel-traffic exposure that comprises not only the pollutants measured in this study but also other pollutants in the complex DE mixture and resuspended coarse particles from road dust, engine debris, and tire debris. The effects of exposure appeared to be larger in the more severe asthmatic group for most outcomes measured. In conclusion, short-term exposure to urban roadside diesel traffic led to consistent and significant reductions in lung function, accompanied by airway acidification and neutrophilic inflammation. Our findings help to explain the epidemiologic evidence on diesel traffic health effects in persons with asthma.