Tropisetron and granisetron are selective serotonin (5-HT3) antagonists that have been proven effective in the prevention of nausea and vomiting in adults and children receiving cancer chemotherapy. This prospective, randomised study was designed to compare the efficacy of the two agents in the prevention of vomiting and nausea in children receiving highly emetogenic chemotherapy for various malignancies. A total of 51 children (mean age: 7.7 +/- 4.8 year) were studied in 133 chemotherapy cycles. In 66 chemotherapy cycles, the children received tropisetron as an antiemetic agent in a dose of 0.2 mg/kg/24 h intravenously and, in 67 cycles, they received granisetron 40 micrograms/kg/24 h intravenously before cytotoxic drug administration during the days they received chemotherapy. The response per 24 h of chemotherapy was defined as complete (no nausea and vomiting), partial (1-4 events of vomiting and/or nausea), and failure (more than 4 events of vomiting and/or nausea). Efficacy of antiemetic therapy was evaluated as acute (Day 1) and overall was based on the worst day during the chemotherapy. Complete control of acute vomiting was achieved in 74% of tropisetron and 88% of granisetron patients (P = 0.04), and complete control of acute nausea in 56% and 82% respectively (p = 0.002). Overall response by means of complete control of both vomiting and nausea during the whole therapy period was 29% of tropisetron group and 55% of granisetron group (p = 0.007). The statistical analysis (depending on the emetogenicity of the chemotherapy cycles) showed increased efficacy of granisetron in highly (grade 3) emetogenic chemotherapy cycles (p = 0.002), whereas there was no difference in the very highly emetogenic cycles (p = 0.7). Also, granisetron was found to be more effective than tropisetron, especially in patients heavier than 25 kg (p = 0.02). The adverse reactions were few and mild. There were no differences in the tolerability of the two antiemetic therapy modalities. In conclusion, granisetron was found to be more effective than tropisetron in controlling nausea and vomiting in children receiving highly emetogenic chemotherapy. This increased antiemetic efficacy of ganisetron might have been related to maximal dose differences according to body weight.