Ondansetron was first synthesised in 1983 and it is estimated that over 4 million patient treatments have been given with ondansetron in clinical practice. Clinical trials in over 15,000 patients have demonstrated that ondansetron is effective and well tolerated in a range of clinical settings, including adults and children and over repeated courses of treatment. Ondansetron is convenient to administer, being effective as a single intravenous dose for the control of acute emesis and as oral therapy on a twice-daily schedule. Recent studies have also shown it to be a cost-effective anti-emetic when the costs of treatment failure of traditional anti-emetics are taken into account. Studies have also demonstrated an improved quality of life with ondansetron compared to treatment with metoclopramide. Ondansetron has also been found to be effective in the prevention or treatment of postoperative nausea and vomiting and other studies are underway to investigate its potential use in other clinical settings. Furthermore, appropriately designed trials are ongoing to evaluate the place of ondansetron in the control of cisplatin-induced delayed emesis. In conclusion, ondansetron has been shown to be a major advance in the control of nausea and vomiting associated with chemotherapy and radiotherapy.