Rationale: Limited prior data suggests an association between traffic-related air pollution and incident asthma in adults. No published studies assess the effect of long-term exposures to particulate matter less than 2.5 µm in diameter (PM2.5) on adult incident asthma. Objectives: To estimate the association between ambient air pollution exposures (PM2.5 and nitrogen dioxide, NO2) and development of asthma and incident respiratory symptoms. Methods: The Sister Study is a US cohort study of risk factors for breast cancer and other health outcomes (n=50,884) in sisters of women with breast cancer (enrollment: 2003-2009). Annual average (2006) ambient PM2.5 and NO2 concentrations were estimated at participants' addresses using a national land-use/kriging model incorporating roadway information. Outcomes at follow-up (2008-2012) included incident self-reported wheeze, chronic cough, and doctor-diagnosed asthma in women without baseline symptoms. Measures and Main Results: Adjusted analyses included 254 incident cases of asthma, 1,023 of wheeze, and 1,559 of chronic cough. For an interquartile range (IQR) difference (3.6 µg/m3) in estimated PM2.5 exposure, the adjusted odds-ratio (aOR) was 1.20 (95% CI=0.99-1.46, P=0.063) for incident asthma and 1.14 (95% CI=1.04-1.26, P=0.008) for incident wheeze. For NO2, there was evidence for an association with incident wheeze (aOR=1.08, 95% CI=1.00-1.17, P=0.048 per IQR of 5.8 ppb). Neither pollutant was significantly associated with incident cough (PM2.5: aOR=0.95, 95% CI=0.88-1.03, P=0.194, NO2: aOR=1.00, 95% CI=0.93-1.07, P=0.939). Conclusions: Results suggest that PM2.5 exposure increases the risk of developing asthma and PM2.5 and NO2 increase the risk of developing wheeze, the cardinal symptom of asthma, in adult women.