We conducted a prospective randomized study to compare granisetron, a 5-hydroxytryptamine-3 receptor antagonist with standard anti-emetics (control group) consisting mainly of metoclopramide, in the prophylaxis of emesis induced by conditioning prior to stem cell transplantation. Fifty-eight patients were evaluable for analysis. The number of emetic episodes expressed in terms of patient-days was significantly lower in the granisetron group than in the control group (P < 0.001). During the first 24 h of conditioning, 27 of the 31 patients (87.1%) in the granisetron group achieved control of emesis with less than three emetic episodes (major < or = ) a day compared with 37.0% in the control group (P < 0.001). The same degree of emesis control was maintained throughout the conditioning period in 51.% of patients in the granisetron group compared with 0% in the control group (P < 0.001). Adverse reactions were observed in 11.4% of patients in the granisetron group and in 25.9% in the control group. None of the events were serious. Based on these data, we conclude that granisetron is superior to standard antiemetics in protecting against the vomiting induced by conditioning for stem cell transplantation.