Epidemiologic evidence linking environmental exposure to polycyclic aromatic hydrocarbons (PAH) with breast cancer is limited. Measurement of DNA adducts formed by aromatic compounds, including PAH, has been carried in breast tissue samples and white blood cells from women with breast cancer and different kinds of controls. However, these studies provide inconsistent results and bias cannot be ruled out. During the 7-year follow-up period, 305 women were diagnosed with first primary breast cancer in the EPIC-Spain cohort, and were compared with a sample of 149 women without breast cancer at recruitment, using a case-cohort approach. Aromatic adducts to DNA from leukocytes collected at recruitment were measured by means of the 32P-postlabelling technique. The relative risk (RR) and 95% confidence interval (CI), adjusted by relevant confounders, were estimated by a modified version of Cox proportional hazards model. There was a significant increased risk for developing breast cancer when DNA adduct concentrations were doubled, with adjusted RR of 1.61 (95% CI 1.29-2.01). The increase in breast cancer risk was observed both for pre- and post-menopausal women. There was a significant interaction with tobacco smoking and body mass index (BMI), with higher effect of DNA adducts on breast cancer risk among smokers and women with normal weight. The results from our study support the hypothesis that factors leading to higher levels of aromatic DNA adducts in white blood cells may be involved in development of breast cancer.