One-hundred and forty-five chemotherapy patients receiving cisplatin- and non-cisplatin-containing regimens participated in an open evaluation of ondansetron, a 5-HT3 receptor antagonist, in the prophylaxis of acute and delayed nausea and vomiting. The study had two groups of patients, one receiving cisplatin-containing regimens (Group A) and the other, non-cisplatin-containing regimens (Group B). There were 51 patients recruited to Group A. Ondansetron was given to these patients at a dose of 8 mg intravenously 15 min before chemotherapy, followed by an intravenous infusion for 24 h (1 mg/h), or 8 mg intravenously 4 and 8 h after the start of cisplatin, followed by 8 mg orally three times/day for 5 days. Ninety-four patients were recruited to Group B: these patients received ondansetron at a dose of 8 mg intravenously immediately before chemotherapy or 8 mg orally 1–2 h prior to it, and 8 mg orally three times/day for 3–5 days. For acute emesis (first 24 h), complete control was achieved in 79.5% of Group A patients and in 78.1% of Group B. For delayed emesis (days 2–5), 79.7% of Group A patients and 84.6% of Group B were completely protected during the entire study period. Nausea was also well controlled with ondansetron; 83.2% (Group A) and 86.5% (Group B) reported only mild nausea or no nausea at all. Ondansetron was effective in the control of both cisplatin- and non-cisplatin-induced emesis and well tolerated without any serious drug-related adverse events.