The management of chemotherapy induced emesis presents a major difficulty in the paediatric population. Ondansetron is a 5-HT3 antagonist and its antiemetic properties have been established in adults receiving chemotherapy. Experience in the treatment of children, however, is limited. There is no standard comparative antiemetic in paediatric practice, so this European, multi-centred study aimed to assess the clinical efficacy and safety of ondansetron in a large cohort of children receiving a range of emetogenic chemotherapy regimens for malignant disease. Two hundred patients were entered into the study, of whom 183 fulfilled entry criteria. Forty per cent (10/25) of patients receiving cisplatin, 68% (27/40) receiving ifosfamide and 70% (83/118) receiving other drugs had less than 3 emetic episodes (vomit or retch) during their worst 24 hours of chemotherapy. The number of days without vomiting or retching during and shortly after receiving chemotherapy was also related to the emetic potential of the agents in the regimen: cisplatin 82/182 (45%), ifosfamide 137/213 (64%) and other agents 430/566 (76%). The control of nausea appeared better than that of emesis in each of the sub-groups analysed. Ondansetron was safe, well tolerated and effective in the prevention of vomiting in children receiving a wide variety of chemotherapy regimens.