['To determine whether estradiol (E(2)) levels after 4 days of ovarian hyperstimulation in leuprolide-downregulated cycles are predictive of embryo quality and pregnancy outcome in oocyte donation.', 'Retrospective analysis of 287 consecutive oocyte donation cycles performed at a university assisted reproduction therapy program between January 1996 and December 2000. Oocyte donors and recipients followed a non-varied standard synchronization regimen. A serum E(2) (day-5 E(2)) was obtained from egg donors the morning after leuprolide acetate downregulation and 4 days after hMG/FSH administration. Day-5 E(2) was categorized into 3 groups: group I <75 pg/ml; group II 75-350 pg/ml, and group III > or =350 pg/ml. All embryo transfers were performed 3 days after oocyte recovery.', 'Elevated day-5 E(2) levels were directly correlated with higher average embryo scores, greater number of oocytes retrieved, and greater number of embryos available for transfer and cryopreservation. Improved pregnancy outcomes were also noted for group III (day-5 E(2) level >350 pg/ml).'] Are serum estradiol levels after 4 days of ovarian hyperstimulation in oocyte donors predictive of embryo quality and clinical outcomes?