OBJECTIVE To investigate the antiemetic effect and tolerability of the 5-hydroxytryptamine3(5-HT3) antagonist ondansetron plus the dopamine D2 antagonist metopimazine versus ondansetron alone in patients receiving platinum-based chemotherapy. METHODS One hundred eleven chemotherapy-naive patients who were scheduled to receive two consecutive courses of platinum-based chemotherapy were randomized between ondansetron 8 mg intravenously (IV) followed by 8 mg orally twice a day plus metopimazine 35 mg/m2 as a 24-hour continuous infusion followed by 30 mg orally four times a day for 4 days, or ondansetron plus placebo. The study used a double-blind, crossover, placebo-controlled design. RESULTS Ninety-four patients completed the crossover. Complete response (CR; no emetic episodes) was obtained on day 1 in 77.7% of the patients who received the combination versus 50.0% of those who received ondansetron alone (P = .00002), and in 51.7% versus 31.0% on days 2 to 6 (P = .0009). The overall CR (days 1 to 6) was 48.9% versus 25.3% (P = .0002). Additionally, significantly less nausea was observed with the combination on day 1 (P = .0002), days 2 to 6 (P = .0001), and days 1 to 6 (P = .00004). Patient preference was 63.6% for the combination and 13.6% for ondansetron alone; 22.7% expressed no treatment preference (P < .0001; therapeutic gain 50.0%; 95% confidence interval [CI], 31.6% to 68.4%). Adverse reactions were mild and without significant differences between the two treatments. CONCLUSIONS Metopimazine plus ondansetron was significantly superior to ondansetron alone, concerning all efficacy parameters assessed, in patients who received platinum-based chemotherapy.