Polycyclic aromatic hydrocarbons (PAH) are ubiquitous in the environment. We hypothesized that early life exposure to PAHs may have particular importance in the etiology of breast cancer. We conducted a population-based, case-control study of ambient exposure to PAHs in early life in relation to the risk of breast cancer. Total suspended particulates (TSP), a measure of ambient air pollution, was used as a proxy for PAHs exposure. Cases (n = 1,166) were women with histologically confirmed, primary, incident breast cancer. Controls (n = 2,105) were frequency matched by age, race, and county of residence to cases. Annual average TSP concentrations (1959-1997) by location were obtained from the New York State Department of Environmental Conservation for Erie and Niagara Counties. Based on the monitor readings, prediction maps of TSP concentrations were generated with ArcGIS 8.0 (ESRI, Inc., Redlands, CA) using inverse distance squared weighted interpolation. Unconditional logistic regression was used to estimate odds ratios and 95% confidence intervals. In postmenopausal women, exposure to high concentrations of TSP (>140 μg/m3) at birth was associated with an adjusted odds ratio of 2.42 (95% confidence interval, 0.97-6.09) compared with exposure to low concentrations (<84 μg/m3). However, in premenopausal women, where exposures were generally lower, the results were inconsistent with our hypothesis and in some instances were suggestive of a reduction in the risk of breast cancer. Our study suggests that exposure in early life to high levels of PAHs may increase the risk of postmenopausal breast cancer; however, other confounders related to geography cannot be ruled out.