OBJECTIVE To determine the relationship between blastocyst score and pregnancy outcome. METHODS Retrospective review of blastocyst transfer in an IVF clinic. METHODS Private assisted reproductive technology unit. METHODS 107 patients undergoing blastocyst culture and transfer of two embryos. METHODS Culture of all pronucleate embryos in sequential media to the blastocyst stage (day 5), followed by transfer of two blastocysts. METHODS Implantation rates, pregnancy rates, and twinning were analyzed. RESULTS When a patient received two top-scoring blastocysts (64% of patients), implantation and pregnancy rates were 70% and 87%, respectively. The twinning rate in this group was 61%. When only one top-quality blastocyst was available for transfer (21% of patients), the implantation and pregnancy rates were 50% and 70%. The twinning rate for this group was 50%. In contrast, when only low-scoring blastocysts were available for transfer (15% of patients), implantation and pregnancy rates were 28% and 44%, and the twinning rate was 29%. No monozygotic twins were observed in this group of patients. CONCLUSIONS The ability to transfer one high-scoring blastocyst should lead to pregnancy rates greater than 60%, without the complication of twins.