Most oocyte donation programs have experienced higher pregnancy rates than usually seen in regular in vitro fertilization (IVF), suggesting that the quality of either the oocytes or the endometrium is superior. To clarify this issue we analyzed the results of transfers of 136 cryopreserved embryos originating either from donated oocytes (18 transfers) or from regular IVF (118 transfers). Transfers of embryos originating from donated oocytes took place after administrating oral estradiol (E2) valerate and vaginal micronized progesterone (P) following a regimen designed to mimic the serum levels of E2 and P observed during the menstrual cycle. Transfers of embryos originating from regular IVF took place either in the natural cycle (53 transfers) or after suppressing ovarian function with a single injection of a gonadotropin-releasing hormone agonist (GnRH-a), Decapeptyl-Retard 3.75 mg, and administering the same hormone replacement regimen (E2/P) used in oocyte donation (65 transfers). Eighteen transfers involving 24 embryos originating from donated oocytes were affected, resulting in six pregnancies (4 ongoing). The ongoing pregnancy rate per transfer was 22%. Seventy-nine embryos originating from regular IVF were transferred (53 transfers) in the natural cycle resulting in six pregnancies (2 ongoing). One hundred three other embryos originating from regular IVF were transferred (65 transfers) after administration of GnRH-a and E2/P resulting in four pregnancies. The pregnancy rate after transfers of embryos originating from regular IVF was 9% per transfer. This was significantly less than the pregnancy rate of 33% per transfer seen after the transfers of embryos originating from donated oocytes.(ABSTRACT TRUNCATED AT 250 WORDS)