High levels of serum estrogens and androgens have been convincingly linked with an increased risk of breast cancer among postmenopausal women. By contrast, the role of blood levels of these hormones in the etiology of premenopausal breast cancer is not well understood. In a case-control study, we sought to examine associations between levels of serum estradiol, sex-hormone binding globulin (SHBG), dehydroepiandrosterone (DHEA), testosterone, androstenedione and progesterone and risk of premenopausal breast cancer. Cases of breast cancer under age 45 were identified using rapid ascertainment systems in Seattle/Puget Sound, Washington and control subjects were identified from the same area through random digit dialing methods. A total of 169 eligible breast cancer cases and 195 control subjects donated blood (either before or six or more weeks after surgery) and were interviewed using a standardized questionnaire. The fully adjusted risk ratios and 95% confidence intervals for the highest versus lowest tertiles of estradiol, according to menstrual cycle phase, were 3.10 (0.8-12.7) for early follicular, 0.54 (0.2-1.7) for late follicular and 0.60 (0.3-1.4) for luteal. Risks for highest versus lowest quartiles of SHBG and androgens were 0.81 (0.4-1.6) for SHBG, 2.42 (1.1-5.2) for DHEA, 1.12 (0.6-2.5) for testosterone, and 1.33 (0.6-2.8) for androstenedione. For luteal progesterone, the RR for the highest versus lowest tertile was 0.55 (0.2-1.4). In summary, we did not find a convincing association between serum SHBG, estradiol, testosterone or androstenedione and premenopausal breast cancer risk. Observed differences between cases and controls subjects in serum levels of DHEA and luteal phase progesterone should be investigated further in large prospective studies.