Ondansetron (Zofron, Glaxo) and tropisetron (Navoban, Sandoz) are selective serotonin (5HT3) antagonists that have proven very effective in the prevention of vomiting and nausea in adults and children receiving cancer chemotherapy. This study compared the efficacy of the two agents in the prevention of vomiting and nausea in children receiving chemotherapy for solid tumors and blood malignancies. A total of 23 children were studied in 205 chemotherapeutic cycles (116 one-day regimens and 89 multiple-day regimens). In 102 chemotherapeutic cycles the children received ondansetron as an antiemetic agent in a dose of 5 mg/m2 30 min before chemotherapy was given and then 4 mg/m2 every 8 h i.v. (group A) and in 103 cycles they received tropisetron in one dose of 0.2 mg/kg 24 h-1 i.v. (max dose 5 mg) 30 min before cytotoxic drugs administration every day they received chemotherapy (group B). The response was defined as complete in the absence of nausea and vomiting per 24 h of chemotherapy, as partial given the presence of 1-4 events of vomiting and/or nausea less than 5 h per 24 h, and as failure if there were more than 4 events of vomiting and/or nausea for more than 5 h per 24 h of chemotherapy. The response of the two groups was studied independently and depending on the degree of emetogenicity of the chemotherapeutic agents, which were divided into mildly, moderately, and highly emetogenic. The comparison of the two groups not taking into consideration the emetogenicity of the chemotherapeutic agents showed that ondansetron was more effective in 1-day regimens (P = .023), whereas the two agents were equally effective in multiple-day regimens (P = .2). The statistical analysis depending on the emetogenicity of the chemotherapeutic agents showed increased efficacy of ondansetron in mild (P = .017) and moderately emetogenic chemotherapeutic agents, whereas there was no difference in the highly emetogenic drug group. Ondansetron is found to be more effective than tropisetron in controlling acute nausea and vomiting in children receiving mild and moderately emetogenic chemotherapeutic drugs, although there is no difference in the efficacy of both antiemetic agents when highly emetogenic drugs are administered.