Ondansetron was given as anti-emetic prophylaxis to 429 children receiving a variety of emetogenic cancer treatments for up to 8 days, in three, open, multicentre, European studies. Children aged between 6 months and 17 years with a variety of tumours and receiving chemotherapy or chemotherapy plus total body irradiation (TBI) were studied. Ondansetron was given intravenously, 5 mg/m2 or 8 mg, according to the surface area of the child, immediately before chemotherapy. Intravenous or oral treatment (2, 4 or 8 mg, according to surface area) was continued 3 times a day during chemotherapy or TBI, and for a further 2 days (non-cisplatin chemotherapy or TBI) or 5 days (cisplatin chemotherapy). The number of vomits and retches (each counting as an emetic episode) were recorded daily, as was an assessment of nausea, which was graded as none (not feeling sick at all), mild (feeling sick) or severe (feeling very sick). Responses were graded according to the number of emetic episodes during the worst 24-hour period. In addition, response was expressed in terms of emesis-free days as a proportion of all ondansetron treatment days. During chemotherapy, 66% of children experienced less than 3 emetic episodes on their 'worst day' and 88% had none or mild nausea. Sixty-eight percent of all ondansetron treatment days (2,131) were free of emesis. Of the patients who were poorly controlled with 'customary' anti-emetics, at least 81% experienced better control with ondansetron. When analysed according to the most emetogenic agent given 36, 59 and 75% of children reported less than 3 emetic episodes on their 'worst day' respectively, during cisplatin, ifosfamide and other less emetogenic chemotherapy. During conditioning for bone marrow transplantation with cyclophosphamide and TBI, 80 and 57% of patients, respectively, experienced less than 3 emetic episodes. The overall incidence of adverse events was low and headache (reported in 4% of patients) was the only event reported by more than 1% of patients. These studies show that ondansetron is a safe, well tolerated and an effective anti-emetic in the treatment of children receiving a wide variety of chemotherapy regimens.