Chemotherapy-induced emesis is dependent not only on individual parameters but also on treatment parameters (type and dose of the cytotoxic drug, combination with other cytotoxic drugs). Cyclophosphamide-based chemotherapy is potentially emetogenic and the emtogenicity is proportionally dependent on the dose. Therefore, the preventive antiemetic treatment must be adapted to this emetogenic risk. A number of studies have assessed the efficacy of ondansetron, a highly selective 5-HT3 receptor antagonist, for the control of 'non cisplatin' chemotherapy. In mild emetogenic regimens, oral ondansetron is more effective than placebo and its efficacy is similar to the classic antiemetic regimen metoclopramide-dexamethasone. Concerning moderately emetogenic chemotherapies, iv ondansetron is highly effective and its effecicay is superior to that of metoclopramide and alizapride. Delayed nausea and vomiting can be controlled by an oral ondansetron treatment. This allows to maintain the good response obtained by the initial antiemetic regimen. With very high doses of cyclophosphamide, as in conditioning chemotherapy and total body irradiation prior to bone marrow transplantation, no optimal antiemetic treatment has still been defined; but the combination of ondansetron with dexamethasone should be used according to the poor control obtained with ondansetron alone. However, studies combining 5-HT3 receptor antagonists with dexamethasone are warranted in order to define the optimal treatment in this particularly emetogenic treatment setting.