OBJECTIVE: The purpose of this study was to investigate the role of leptin on reproductive hormones and ovulation. STUDY DESIGN: The BioCycle Study (2005-2007) followed 259 healthy premenopausal women not using hormonal contraceptives for ≤2 menstrual cycles (n = 509 cycles). Serum leptin, estradiol, progesterone, luteinizing hormone (LH), follicle-stimulating hormone, and testosterone were measured ≤8 times per cycle. The association of time-varying leptin and reproductive hormones over the cycle was estimated with the use of linear mixed models that were adjusted for percent body fat and age with inverse probability weighting for time-varying physical activity, caloric intake, and other reproductive hormones. The odds ratio for sporadic anovulation (n = 42 cycles) was estimated with the use of generalized linear models that were adjusted for percent body fat and age. RESULTS: Geometric mean serum leptin levels increased from menses to the late luteal phase (16.7-20.4 ng/mL; P < .01), with a mid-cycle peak (21.7 ng/mL) at the time of the LH surge (P