Although significant advances have been made in the treatment of malignant diseases, many agents cause nausea and vomiting severe enough to cause patients to be hospitalised or to refuse further treatment. Several classes of anti-emetics are currently used, but are only partially effective and are often associated with side effects such as extrapyramidal reactions. Ondansetron, a specific 5-HT3 antagonist, has been fully evaluated in the clinic, both as an intravenous and oral presentation, and in open studies in patients receiving non-cisplatin chemotherapy regimens it was highly effective in controlling acute and delayed emesis -- more than 90% of patients had a complete or major response to treatment. In three randomised, double-blind studies comparing ondansetron and metoclopramide, ondansetron was found to be superior in the control of both emesis and nausea. Ondansetron was also shown to be safe and well tolerated; in particular, no extrapyramidal reactions were reported.