In a multicenter trial, we evaluated the antiemetic efficacy of ondansetron, a selective serotonin type 3 (5-HT3) receptor antagonist, in 42 adult chemotherapy-naïve patients receiving a multiple-day cisplatin regimen (20-40 mg/m2 per day for 4-5 days). Thirty-one patients received 3 daily doses of ondansetron (0.15 mg/kg) given intravenously every 6 hours (first dose 30 minutes prior to cisplatin administration); 11 additional patients received an identical dosage and schedule except that a fourth daily dose was added 17.5 hours after cisplatin administration. No other antiemetics were administered. Forty patients were evaluable for efficacy response. Thirteen patients (33%) had no vomiting at any time during the 5-day study. When emetic episodes were evaluated on a daily basis, complete protection (zero emetic episodes) ranged from 50-75%, and major protection (less than or equal to 2 emetic episodes) ranged from 65-93%. The majority of therapy failures occurred on days 3 and 4. Side effects were minor and transient; no extrapyramidal side effects were observed. Ondansetron appears to be a safe and effective antiemetic when administered during a multiple-day cisplatin-containing chemotherapy regimen.