The efficacy of the serotonin antagonist ondansetron (GR38032F, Glaxo) was evaluated in the prevention of nausea and vomiting induced by combinations containing cyclophosphamide (CTX) greater than or equal to 600 mg/m2 IV day. At their first treatment course, 55 patients (10 males, 45 females) median age 55 years (range 31-76) were given ondansetron 8 mg orally tds for a minimum of 3 to a maximum of 5 days. 54 patients were evaluable. Complete and major control of acute (day 1) emesis was observed in 94.5% of patients and acute nausea was graded as absent or mild in 83.3% of cases. Complete and major control of emesis improved on subsequent study days from 96.1% on study day 2 to 100% on study day 5. Side effects were mild. Ondansetron is a safe and effective antiemetic drug.