The proposed study is the establishment of a large cohort of women aged 20-50 in whom contraceptive practices will be determined. In addition, known risk factors in breast cancer will be recorded along with the prevalence and incidence of diseases associated with oral contraceptive use. The population evaluated will be constituted from physicians' wives and women physicians who graduated from medical school between 1948 and 1973. Of the approximate 180,000 persons in this category it is estimated that approximately 81,000 women willing to be followed will be identified. The basic instrument will be a self-completed questionnaire. Appropriate validation of the quality of the data obtained will be undertaken along with assessment of the response patterns in specific demographically identified groups to determine the representativeness of the responders. Relative risks for side effects of various contraceptive practices will be determined but the primary reason for establishing the cohort is in anticipation of a long range prospective study of the relationship of oral contraceptives and breast cancer. Sufficient numbers in the cohort will be available to identify either an excess or a protective causal association between oral contraceptives and breast cancer.