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RATIONALE Ultrafine particle (UFP; particles < 0.1 μm in diameter) concentrations exhibit high spatiotemporal variability, thus individual-level exposures and health risks are difficult to estimate. OBJECTIVES To determine the effects of recent UFP exposures on respiratory health outcomes in children and determine if children with asthma are at increased risk. METHODS Personal sampling of UFPs was completed by adolescents in combination with repeated personal spirometry measurements and ecological momentary assessment of respiratory symptoms (wheeze, cough, shortness of breath). We assessed the association between UFP exposures every 30-mins up to 150 min prior to measuring FEV1 (forced expiratory volume in 1 s), PEF (peak expiratory flow), and respiratory symptoms using mixed-effects models, and interaction with asthma diagnosis. MEASUREMENTS AND MAIN RESULTS Participants (n = 105; 43% with asthma) completed an average of 11 spirometry measurements and 16 symptom responses throughout sampling. After adjustments (maternal education, physical activity, season, and distance to nearest roadway), a 10-fold increase in UFP exposure was significantly associated with a 0.04 L decrease (95% CI: -0.07, -0.001) in FEV1 90 minutes later. Asthma status modified this association in which asthmatics experienced significantly lower FEV1 values in response to UFP exposures 30 minutes prior compared to non-asthmatics. We found a significant increase in the odds of reporting a respiratory symptom 30 mins following increased UFP exposure (OR: 1.8, 95% CI [1.00, 3.00]). CONCLUSION Greater UFP exposure conferred deleterious effects on lung function and respiratory symptoms within 90 minutes of exposure, and was more pronounced among asthmatics.