Background: Proximity to roadways increases the risk of asthma in developed countries; however, relatively little is known about this relationship in developing countries, where rapid and uncontrolled growth of cities has resulted in urban sprawl and heavy traffic volumes. Objective: We sought to determine the effect of distance from a heavily transited avenue on asthma symptoms and quantitative respiratory outcome measures in a periurban shantytown in Lima, Peru. Methods: We enrolled 725 adolescents aged 13 to 15 years who were administered a survey on asthma symptoms and measured spirometry, response to allergy skin testing, and exhaled nitric oxide (eNO). We calculated distances from the main avenue for all households and measured indoor particulate matter in 100 households. We used multivariable regression to model the risk of asthma symptoms, risk of atopy, eNO levels, and FEV1/forced vital capacity ratio as a function of distance. Results: Compared against 384 meters, the odds of current asthma symptoms in households living within 100 meters increased by a factor of 2 (P < .05). The odds of atopy increased by a factor of 1.07 for every 100-meter difference in the distance from the avenue (P = .03). We found an inverse relationship in prebronchodilator FEV1/forced vital capacity and distance to the avenue in female subjects (P = .01) but not in male subjects. We did not find an association between eNO or household particulate matter levels and distance. Conclusion: Living in close proximity to a high-traffic-density avenue in a periurban community in Peru was associated with a greater risk of asthma symptoms and atopy. Regulation of mobile-source pollutants in periurban areas of developing countries might help reduce the burden of asthma symptoms and atopy. © 2010 American Academy of Allergy, Asthma & Immunology.