We evaluated the efficacy and safety of oral ondansetron, a selective antagonist of 5-HT3 receptors, for the treatment of nausea and vomiting associated with cyclophosphamide-based chemotherapy (> 500 mg/m2). In this trial 324 chemotherapy-naive cancer patients, mostly females with breast cancer, were randomized to receive either placebo or ondansetron 1 mg, 4 mg, or 8 mg three times per day for 3 days. There were no differences in the doses of cyclophosphamide, doxorubicin, and methotrexate between the study groups. All ondansetron dose groups were superior to the placebo control group (p < .001) for all measured efficacy parameters (complete response, number of emetic episodes, therapeutic failures, need of rescue antiemetics). No emetic episodes were reported by 9 (12%), 29 (37%), 48 (64%), and 47 (66%) of the placebo patients and the 1-mg, 4-mg, and 8-mg dose of ondansetron patients, respectively. Nausea was reduced and food intake was improved for all the ondansetron groups. A more severe emetic response was observed in patients receiving cyclophosphamide and doxorubicin combination chemotherapy. In this subgroup of patients, 66%, 38%, 25%, and 16% of the placebo group and 1-mg, 4-mg, and 8-mg ondansetron patients, respectively, required rescue antiemetics. No significant toxic effects were observed in this study. A higher incidence of headaches and gastrointestinal complaints (constipation, abdominal pain) were observed in the three ondansetron groups. In conclusion, oral ondansetron is an effective and well-tolerated antiemetic treatment in the management of cancer patients receiving ambulatory cyclophosphamide-based chemotherapy. These results support the view that serotonin and 5-HT3 receptors play an important role in cyclophosphamide-induced nausea and vomiting.