Ondansetron is a selective 5-HT3 receptor antagonist which has previously been reported in the Journal to be a promising new agent for use as prophylaxis against nausea and vomiting caused by chemotherapy and radiotherapy. Since the publication of this original review, further studies have been published that show ondansetron to be an effective antiemetic agent in patients receiving chemotherapy and radiotherapy. Several studies have shown ondansetron to be a more effective antiemetic agent than high-dose metoclopramide in patients with emesis induced by high- and low-dose cisplatin treatment, and noncisplatin chemotherapy-induced emesis. The drug as mono-therapy does not appear to offer any advantage over alternative therapies against delayed high-dose cisplatin-induced nausea and vomiting; however, extremely limited data suggest that ondansetron plus dexamethasone may be useful in this indication. Trials have shown combination therapy with ondansetron and dexamethasone to be significantly more effective than both ondansetron monotherapy and a standard antiemetic regimen comprising metoclopramide, dexamethasone and diphenhydramine against acute high-dose cisplatin-induced emesis. Results from a number of small scale trials suggest that ondansetron may be an effective treatment for chemotherapy-induced emesis refractory to conventional antiemetic therapy. Ondansetron also appears to be more effective against refractory emesis induced by noncisplatin chemotherapy than that induced by cisplatin chemotherapy. Several trials have shown ondansetron to be more effective than placebo as prophylaxis against postoperative nausea and vomiting; a further trial has shown single-dose ondansetron to be significantly more effective than single-dose droperidol or metoclopramide in this indication. In addition, several trials have shown ondansetron to be more effective than placebo as treatment for nausea and vomiting that has commenced postoperatively. The overall incidence of adverse events in ondansetron recipients during chemotherapy-induced emesis studies was 36%. Headache and constipation are the most common adverse events during ondansetron therapy. Thus, recent data affirms the efficacy of ondansetron in the treatment of acute chemotherapy-induced nausea and vomiting and shows it to be especially efficacious when combined with dexamethasone. It appears that the drug will also have a substantial role in the prophylaxis and treatment of postoperative nausea and vomiting.