Epidemiologic evidence suggests obese premenopausal women experience a reduced risk of breast cancer. The mechanism underlying this protection is not fully understood although it is well documented that abdominal obesity may impair ovulatory function and reduce gonadal steroidogenesis. We measured levels of several metabolic markers that are modified by obesity [measured by body mass index (BMI, (weight (kg)/height (m2)))] and play a role in the reproductive axis, including, leptin, insulin-like growth factor I (IGF-I), insulin-like growth factor binding protein 3 (IGFBP3), C-peptide and prolactin in 233 cases and 251 controls participating in a retrospective study of breast cancer in young women conducted in the Seattle/Puget Sound region between 1990 and 1992. Consistent with the finding of a reduced risk with increasing BMI, risks declined with leptin levels, although to a lesser degree with odds ratios (OR) for the highest vs. lowest quartile of BMI=0.34 (95% C.I. 0.3-0.8) and for leptin=0.71 (95% C.I. 0.5-1.3). IGF-I, IGFBP3, C-peptide and prolactin were not related to breast cancer risk in a dose-dependent manner. With the possible exception of leptin, our findings do not suggest that these markers explain the breast cancer protection provided by obesity in premenopausal women.