Context: Previous studies have suggested that estrogen levels may be higher in African-American women (AAW) compared with Caucasian women (CW), but none have systematically examined estrogen secretion across the menstrual cycle or in relation to other reproductive hormones. Objective: The objective of the study was to compare estradiol (E2), progesterone (P), gonadotropins, androstenedione (a’dione), inhibins, and SHBG levels between AAW and CW across the menstrual cycle. Design, Setting, and Subjects: Daily blood samples were collected from regularly cycling AAW (n = 27) and CW (n = 27) for a full menstrual cycle, and serial ultrasounds were performed. Main Outcome Measures: Comparison of E2, P, LH, FSH, SHBG, inhibin A, inhibin B, and a’dione levels. Results: AAW and CW were of similar age (27.2 ± 0.6 yr, mean ± sem) and body mass index (22.7 ± 0.4 kg/m(2)). All subjects grew a single dominant follicle and had comparable cycle (25-35 d) and follicular phase (11-24 d) lengths. E2 levels were significantly higher in AAW compared with CW (P = 0.02) with the most pronounced differences in the late follicular phase (225.2 ± 14.4 vs. 191.5 ± 10.2 pg/ml; P = 0.02), midluteal phase (211.9 ± 22.2 vs.150.8 ± 9.9, P < 0.001), and late luteal phase (144.4 ± 13.2 vs. 103.5 ± 8.5, P = 0.01). Although LH, FSH, inhibins A and B, P, a'dione, and SHBG were not different between the two groups, the a'dione to E2 ratio was lower in AAW (P < 0.001). Conclusions: Estradiol is higher in AAW compared with CW across the menstrual cycle. Higher estradiol in the face of similar androstenedione and FSH levels suggests enhanced aromatase activity in AAW. Such differences may contribute to racial disparities in bone mineral density, breast cancer, and uterine leiomyomas.