The efficacy of the serotonin antagonist ondansetron (GR 38032F) was evaluated in the prevention of nausea and vomiting induced by CMF chemotherapy in 29 breast cancer patients. At their first treatment course of CMF, all given IV on day 1 q 21 days, patients were given oral antiemetic treatment as follows: ondansetron 8 mg, 2 h prior to CMF, repeated after 5 and 10 h the day of chemotherapy and then 8 mg tds for a minimum of 3 days to a maximum of 5 days following chemotherapy. At first course of CMF, complete protection from emesis and nausea was observed in 86.2% and 62% of patients, respectively. At subsequent CMF courses with ondansetron, complete control of emesis was observed in 80% of patients. Side effects were mild and no dystonic reactions were observed. Ondansetron represents an effective, safe, and easily administered outpatient regimen. The addition of a corticosteroid to ondansetron could further improve control of CMF-induced emesis.