Chemotherapy-induced emesis is a major problem in the treatment of childhood mali-gnancies. Ondansetron, a highly selective 5-HT3 receptor antagonist has been used in the prevention of chemotherapy induced emesis and nausea in children. In this pro-spective study ondansetron was given as anti-emetic prophylaxis to 60 children recei-ving 81 chemotherapy courses for up to 8 days each. Children were suffering from a variety of malignant tumours and their ages ranged between 10 months and 15 years. Ondansetron was given intravenously, 5 mg/m2, 15 minutes before, 4 and 8 hours following each dose of chemotherapy and was continued during the whole course of chemotherapy. The number of vomits and retching (each counting as an emetic episode) were recorded daily. The degree of nausea was assessed as follows: a) none (not feeling sick at all), b) mild (feeling sick) and c) severe (feeling very sick). The chemotherapeutic regimens were categorized according to their emetogenic potency as groups I (regimens containing cisplatin), II(noncisplatin containing regimens) and III (24 hour infusion of MTX, ARA-C). Responses were graded according to the number of emetic episodes during the worst-24-hour period: major response (1-2 emetic episodes/mild nausea), complete response (0 emetic episodes/no nausea).Table