To determine the comparative anti-emetic efficacy of ondansetron and granisetron in patients undergoing bone marrow transplantation, we performed a double-blind, randomized trial in pediatric and adult patients receiving transplants at the University of Minnesota. The results in 187 patients stratified by age (<18 years, n = 51; > or =18 years, n = 136) were analyzed. The average number of emetic episodes in the entire group from day -7 to 2 was 0.86/day for patients receiving ondansetron and 0.73/day for those receiving granisetron (p = 0.32). No differences were noted between the two drugs in total days of complete or major control of emesis or in the number of requests for additional drugs to alleviate symptoms of nausea. The use of total-body irradiation-containing conditioning regimens was associated with a decreased number of emetic episodes compared with regimens of chemotherapy alone. Perceived nausea was evaluated using a nausea scoring system, and no differences were apparent between the granisetron and ondansetron groups; however, reported nausea was significantly higher in females (p<0.01) and in the adult population (p = 0.05). We conclude that both ondansetron and granisetron provide good control of nausea and vomiting experienced with conditioning regimens for bone marrow transplantation. The relative cost of the drugs within an institution must be considered in developing standard anti-emetic regimens for bone marrow transplantation.