High-dose melphalan has been gaining recognition as a highly emetogenic agent used in hematopoietic stem cell transplantation (HSCT). The aim of this retrospective study was to elucidate the efficacy of aprepitant in preventing high-dose melphalan-induced emesis. Sixty patients who received melphalan (70 mg/m(2)/day, 2 days) and fludarabine (125 mg/m(2)/day, 5 days) as conditioning for allogeneic HSCT for hematological malignancies, and who received ondansetron and methylprednisolone as an antiemetic prophylaxis, were eligible. Twenty of these 60 patients also received aprepitant for 5 days (aprepitant group); the remaining 40 patients served as a control. The rates of complete response (CR), defined as no emesis without rescue medications, and complete protection (CP), defined as no emesis with or without rescue medications, were assessed between the two groups. The observation period was 12 days from the first day of melphalan administration. The CR and CP rates were significantly higher in the aprepitant group than in the control group during the observation period (35 % versus 10 %, P < 0.05; 85 % versus 33 %, P < 0.001; respectively). These results suggest that aprepitant in combination with ondansetron and steroid effectively ameliorates nausea and vomiting caused by the high-dose melphalan-based conditioning for allogeneic HSCT.