The primary objective of this contract is to provide for the conduct of an epidemiologic study of the risk of breast cancer among women, ages 35- 64, in relation to oral contraceptive use. Five Fields Centers shall participate in the conduct of a multi-center, population-based, concurrent case-control study to elucidate effects of: 1) current and lifetime OC use; 2) timing of OC use relative to other reproductive events; 3) differential effects of contraceptive estrogens and progestins; and 4) possible modification of OC effects by subsequent estrogen or hormone replacement therapy on the risk of breast cancer. A sufficient number of breast cancer cases and controls shall be enlisted to provide for the detection of a small overall relative risk of breast cancer (approximately a 20% increase or decrease) in oral contraceptive users compared to non-users. In addition, the sample size shall be large enough to provide for subgroup analyses, based on possible differences in risk that may exist for various subgroups. Subgroups shall be defined in terms of race, family history of breast cancer, parity and other reproductive characteristics of women, and other suspected breast cancer risk factors. A secondary objective of this contract is to examine biological differences between cases, which may be associated with exogenous hormone use.