Background :The conventional four-times-daily intravenous conditioning regimen of busulfan for hematopoietic stem cell transplantation (HSCT) has shown limitations, such as low patient compliance, inconvenient preparation and administration, and stability. In this study, we compared the efficacy and safety between four-times-daily regimen of busulfan and once-a-day regimen of busulfan in adult patients undergoing allogeneic HSCT. Methods :We retrospectively analyzed the electronic medical records until September 2019 of patients who were administered busulfan once a day (ivBU1 group) and four times daily (ivBU4 group) between January 2016 to December 2016. Primary outcomes were 1-year survival, disease-free survival, relapse rate, non-relapse mortality, the incidence of graft-versushost disease (GVHD), and neutrophil and platelet recovery time. The secondary outcomes were cytomegalovirus (CMV) infection and veno-occlusive disease (VOD) incidence. Results :Comparison of the ivBU1 group (n=75) and the ivBU4 group (n=59) showed that the 1-year survival rate was 75.1% vs 62.6% (p=0.127) and the 1-year disease free survival rate