BACKGROUNDPalonosetron, a highly selective and potent 5‐HT3 receptor antagonist with a strong binding affinity and a long plasma elimination half‐life (approximately 40 hours), has shown efficacy in Phase II trials in preventing chemotherapy‐induced nausea and vomiting (CINV) resulting from highly emetogenic chemotherapy. The current Phase III trial evaluated the efficacy and safety of palonosetron in preventing acute and delayed CINV after moderately emetogenic chemotherapy.METHODSIn the current study, 592 patients were randomized to receive a single, intravenous dose of palonosetron 0.25 mg, palonosetron 0.75 mg, or dolasetron 100 mg, 30 minutes before receiving moderately emetogenic chemotherapy. The primary efficacy endpoint was the proportion of patients with a complete response (CR; defined as no emetic episodes and no rescue medication) during the first 24 hours after chemotherapy. Secondary endpoints included assessment of prevention of delayed emesis (2–5 days postchemotherapy).RESULTSIn the current study, 569 patients received study medication and were included in the intent‐to‐treat efficacy analyses. CR rates during the first 24 hours were 63.0% for palonosetron 0.25 mg, 57.1% for palonosetron 0.75 mg, and 52.9% for dolasetron 100 mg. CR rates during the delayed period (24–120 hours after chemotherapy) were superior for palonosetron compared with dolasetron. Adverse events (AEs) were mostly mild to moderate and not related to study medication, with similar incidences among groups. There were no serious drug‐related AEs.CONCLUSIONSA single dose of palonosetron is as effective as a single dose of dolasetron in preventing acute CINV and superior to dolasetron in preventing delayed CINV after moderately emetogenic chemotherapy, with a comparable safety profile for all treatment groups. Cancer 2003. © 2003 American Cancer Society.