OBJECTIVE: This study evaluated the pregnancy and implantation rates in fresh IVF-ET cycles or frozen-thawed ET (F-ET) cycles based on serum estradiol concentrations of controlled ovarian hyperstimulation (COH). METHODS: Clinical outcomes of 1,565 cycles of fresh IVF-ET with COH and 670 cycles of F-ET were retrospectively analyzed. Serum estradiol levels on the day of human chorionic gonadotropin (hCG) administration were categorized into Group-A (1,000~ 2,000 pg/ml), Group-B (2,000~3,000 pg/ml), Group-C (3,000~4,000 pg/ml) and Group-D (> 4,000 pg/ml). Clinical pregnancy (CPR), implantation (IR) and delivery rates (DR) were compared among four groups subdivided into younger (< 35 years) and older (> or = 35 years) women. Statistical analysis was performed by Student's t-test and chi-square test. RESULTS: Overall clinical outcomes with fresh IVF-ET and F-ET cycles were similar: 41.2% vs 44.8% of CPR, 18.8% vs 19.6% of IR, and 33.2% vs 34.5% of DR, respectively. There were no significant differences in the clinical outcomes of all four groups between fresh IVF-ET and F-ET cycles of younger women according to the estradiol levels. However, the clinical outcomes of F-ET cycles of older women in Group-D were significantly higher than those of fresh IVF-ET cycles (51.3% vs 25.0% of CPR*, 18.6% vs 9.9% of IR and 33.3% vs 19.4% of DR; *p<0.05). CONCLUSION: Our results demonstrated that supraphysiological levels of estradiol during COH in fresh IVF-ET cycles of older women (> or = 35 years) may be detrimental to implantation environments of endometrium and clinical outcomes, which could be improved by F-ET cycles.