OBJECTIVE: To examine the results of 7 years of thawed ET during natural or controlled cycles using exogenous steroids. DESIGN: Retrospective evaluation to compare implantation and pregnancy rates with two protocols for transfer of cryopreserved-thawed pre-embryos. SETTING: Tertiary care academic center. PATIENTS: From January 1987 to December 1993, 521 patients who were < 40 years of age underwent 628 thawed embryo transfers. MAIN OUTCOME MEASURE: Pregnancy and implantation rates per thawed embryo transfer cycle. RESULTS: A total 1,987 pre-embryos survived the thawing process and were used in 628 thaw-transfer cycles. Transfer was performed in a natural cycle 2 days after the LH peak or on day 17 of a programmed cycle using a GnRH-agonist and hormone replacement therapy protocol; 182 pregnancies were established (182/628; 29%). Similar pregnancy rates were seen in the natural cycle (112/398; 28%) and the programmed cycle (70/230; 30%). The implantation rates were similar in the two methods of transfer cycles (11.9% versus 10.3%, natural versus programmed cycle). There were no significant differences in clinical or ongoing pregnancy rates in a natural or programmed cycle, correcting for the number of cryopreserved-thawed pre-embryos transferred. Patient's age at the time of freezing and the number of cryopreserved-thawed pre-embryos transferred are more important determinants of pregnancy than the type of cycle in which transfer occurs. CONCLUSION: Transferring cryopreserved-thawed pre-embryos in a natural or programmed cycle yields similar pregnancy results.