A total of 332 postpartum (pp) lactating Friesian cows allocated in two treatment groups at a commercial dairy farm were used to study the effect of a modified Ovsynch protocol supplemented with exogenous estradiol (E2) on fertility in lactating dairy cows during the summer season. All cows were injected with gonadotrophin releasing hormone (GnRH), followed 7 days later by an injection of prostaglandin (PGF2α). Then cows were randomly assigned to receive GnRH and TAI 72 h after PGF2α (CO-72; control group) or E2, GnRH and TAI at 48, 56 and 72 h, respectively after PGF2α (OV-56+E2). Estrus response for cows in the OV-56+E2 (59.8%) group was higher (P < 0.05) than for cows in the CO-72 (40.2%) group. Pregnancy per insemination (P/AI) at two diagnoses (days 30 to 35 and confirmed at days 45 to 50 after AI) were similar between the two treatment groups, but were higher (P < 0.05) for cows that showed estrus (59.8 and 41.2%, respectively) than for cows that did not show estrus (45.5 and 7.9%, respectively). Moreover, P/AI at the two diagnoses were higher (P < 0.05) for cows with a body condition score (BCS) >2.5 (60.1 and 36.4%, respectively) compared to those with BCS <2.5 (44.3 and 16.5%, respectively). Pregnancy losses were higher (P < 0.05) for cows that did not show estrus (82.6%) or with BCS <2.5 (62.8%) than for cows that showed estrus (31.0%) or with a BCS >2.5 (39.5%). No differences were observed in P/AI or pregnancy losses between primiparous and multiparous cows. Results indicate that the OV-56+E2 synchronization protocol resulted in a higher expression of estrus than the CO-72 without improving fertility as measured by P/AI and pregnancy losses. However, cows with high BCS had higher estrus expression, pregnancy rates, and lower pregnancy losses than cows with low BCS during the summer season.