Objective: To analyze the role of serum LH and of E 2 secretion on IVF-ET outcome in patients stimulated with FSH. Design: Using data from three studies, we analyzed ovarian response to FSH and IVF-ET outcome from two standpoints: [1] serum LH and [2] serum E 2 on the day of hCG administration divided by the number of retrieved oocytes. Setting: Twenty-six academic and private clinical centers. Patient(s): Three hundred twenty-three ovulatory patients eligible for IVF-ET. Intervention(s): Patients were treated with a GnRH agonist and FSH and underwent IVF-ET. Main Outcome Measure(s): Follicular development, embryos, and pregnancy rate (PR). Result(s): Serum LH levels did not correlate significantly with number of oocytes retrieved, E 2 -oocyte ratio, fertilization rate, or PR. Five patient subpopulations were identified on the basis of E 2 -oocyte ratios: 0 to 70 (A), 70 to 140 (B), 140 to 210 (C), 210 to 280 (D), and >280 (E) pg/mL per oocyte (conversion factor to SI unit, 3.671). Pregnancy rates were significantly different, i.e., 5.3%, 31.3%, 18.1%, 23.9%, and 20.4% for groups A, B, C, D, and E, respectively. Groups were not different in terms of baseline characteristics, number of follicles, fertilization rates, and number of embryos transferred. Conclusion(s): Patients with very low levels of LH respond to FSH alone as well as those with higher LH. The E 2 -oocyte ratio is a strong index of success rate.