Background. Worldwide, breast cancer incidence continues to rise in all age groups. The cause of increased risk of breast cancer is not completely known. A twin study shows approximately two-thirds of breast cancer related to environmental factors. Methods. To validate the smoking questionnaire in the Shanghai Women's Health Study (SWHS), a cross-sectional study was conducted. A total of 571 urine samples was randomly selected from the 88% subjects with baseline urine samples available in the SWHS and assayed for cotinine. To evaluate the association between environmental factors—smoking and cooking fumes—and breast cancer, a population-based prospective cohort study was conducted nested in the Shanghai Women's Health Study (SWHS) of 74,942 female permanent residents in seven communities in urban Shanghai. During the follow-up to December, 2003, a total of 396 women were diagnosed with primary breast cancer. An additional 2097 active smokers including 6 cases were excluded for analysis of secondhand smoke (SHS). Lifetime exposure to tobacco smoke and cooking fumes was obtained by questionnaire at baseline and the first follow-up. Results. In the validation study, urinary cotinine was strongly associated with both active smoking and SHS levels in our questionnaire and a significant dose-response relationship existed between baseline urinary cotinine level and cigarettes per day smoked by women and by their husbands at baseline and hours per day exposed at home at the first follow-up. Sensitivity and specificity for self-reported current smokers were high as were the positive and negative predictive values. In this prospective cohort study, breast cancer risk was associated with exposure to tobacco smoke, mainly secondhand smoke (SHS). The association between active smoking and breast cancer seemed to be modified by exogenous hormones and history of reproductive diseases. Breast cancer risk among non-smokers increased with an increase in duration-weighted hours per day of SHS and cumulative SHS in hour-years and this association was synergistic by exogenous hormones. Also breast cancer risk was associated with long-term exposure to cooking fumes from gas use and with increase in duration of gas use and the association is independent of effects of smoking and other risk factors on breast cancer. Conclusions. This dissertation suggests environmental factors contribute to increased risk of breast cancer among Shanghai women and the smoking questionnaire in the SWHS is sufficiently valid as a method for obtaining exposure information.