Objective: To evaluate the influence of different E2 supplementation doses dur ing the luteal phase on implantation and pregnancy rates in women undergoing int racytoplasmic sperm injection (ICSI) cycles. Design: Prospective, randomized stu dy. Setting: A private IVF unit. Patient(s): One hundred sixty-six women younge r than 40 years who were undergoing IVF with long protocol controlled ovarian hy perstimulation (COH). A total of 231 cycles were investigated. Group 1 (P only) included 80 cycles, group 2 (P and 2 mg of E2) included 73 cycles, and group 3 ( P and 6 mg of E2) included 78 cycles. Intervention(s): Supplementation in the lu teal phase with different doses of E2 (0, 2, or 6 mg/d). Main Outcome Measure(s) : Serum E2 and P levels in the late luteal phase, and implantation rate and preg nancy rate (PR) were documented. The data were analyzed with regard to the entir e study population and further stratified according to the E2 dose used. Result( s): Significantly higher implantation rate and PR were recorded in those who rec eived low dose E2 supplementation compared with no substitution (PR 23.1%vs. 32 .8%). The best implantation and pregnancy results were found significantly in t he group with high dose E2 supplementation (PR 51.3%). Conclusion(s): For women treated with a long GnRH analogue protocol for COH, addition of a high dose of E2 to daily P supplementation significantly improved the IVF-embryo transfer re sults. Objective: To evaluate the influence of different supplements on dur ing the luteal phase on implantation and pregnancy rates in women undergoing int racytoplasmic sperm injection (ICSI) cycles. Design: Prospective, randomized stu dy. Setting: A private IVF unit. Patient ( s): One hundred sixty-six women younge r than 40 years who were undergoing IVF with long protocol controlled ovarian hy perstimulation (COH). Group 1 (P only) included 80 cycles, group 2 ( P and 2 mg of E2) included 73 cycles, and group 3 (P and 6 mg of E2) included 78 cycles. Intervention (s): Supplementation in the lu teal phase with different doses of E2 (0, 2, or 6 mg / d). Main Outcome Measure (s): Serum E2 and P levels in the late luteal phase, and implantation rate and preg nancy rate (PR) were documented. The data were analyzed with regard to the entir e study population and further stratified according to the E2 dose used. Result (s): Significantly higher implant ation rate and PR were recorded in those who who recived low dose E2 supplementation compared with no substitution (PR 23.1% vs. 32.8%). The best implantation and pregnancy results were found significantly in t he group with high dose E2 supplementation ( PR 51.3%). Conclusion (s): For women treated with a long GnRH analogue protocol for COH, in addition to a high dose of E2 to daily P supplementation significantly improved the IVF-embryo transfer re sults.