Ondansetron, a 5HT3 antagonist, was given to 20 children aged 4 to 18 years who were undergoing treatment with the Australian and New Zealand Childhood Cancer Study Group Acute Lymphocytic Leukaemia (ALL) Study V Protocol. The study was open, dose ranging, and noncomparative, and designed to evaluate safety and efficacy of ondansetron in preventing nausea and vomiting caused by cyclophosphamide intravenous (IV) 1,000 mg/m2 day 1, and cytarabine IV subcutaneously (SC) 75 mg/m2 on days 2 to 5. Ten patients were given ondansetron 5 mg/m2 IV (group A) and subsequently another 10 patients were given ondansetron 3 mg/m2 IV (group B). Oral ondansetron was given for 14 doses, at the same dosage for both groups, commencing simultaneously with the IV infusion and continuing at 8 hourly intervals, ie, until day 5. The oral dose was based on surface area with the following schedule: 0.3 to 0.6 m2, 2 mg; 0.6 to 1 m2, 3 mg; and greater than 1 m2, 4 mg. Vomiting on the first day of chemotherapy was reported in group A by one patient and by one patient in group B. Vomiting during days 2 to 5 was reported by two group-A patients and by three group-B patients. Nausea was recorded on day 1 by one patient in group A, and two in group B, and on days 2 to 5 by three patients in group A, and by seven in group B. All patients were alert during treatment with ondansetron and there was no dystonia. There were no changes in renal function or hematology values that could be ascribed to the study drug. Transient elevations in bilirubin and liver enzymes were observed. We conclude that our results indicate that ondansetron is a safe and extremely effective single-agent antiemetic with minimal side effects, when administered both IV and orally.