A North European, randomised, double-blind study, comparing a loading-dose of ondansetron of 5 mg/m2 with 10 mg/m2, administered intravenously before highly emetogenic chemotherapy, was carried out in 187 chemotherapy-naïve children. In the first 24 h, both groups received further ondansetron intravenously at a dose of 5 mg/m2 8-hourly. Thereafter, ondansetron was given at an oral dose of 4 or 8 mg depending on the surface area of the child, three times a day and continued for at least 3 days after the last day of chemotherapy. There was no difference in the control of emesis between the two groups. Ondansetron provided good control of emesis and nausea on day 1 with 71-72% of patients experiencing two or fewer emetic episodes (complete or major responders) and 90-86% of patients reporting nausea as none or mild. There was also no difference in the efficacy of the treatment arms in the control of emesis and nausea on subsequent days of the study period. Both anti-emetic regimens were well-tolerated.