PURPOSE The effectiveness of a dexamethasone (DEX)-free regimen for chemotherapy-induced nausea and vomiting (CINV) prophylaxis in patients receiving highly emetogenic chemotherapy (HEC) is not known. METHODS This was a double-blind, phase III trial designed to show the noninferiority of a DEX-free regimen (olanzapine, palonosetron, and fosaprepitant [OPF]) compared with the DEX-containing regimen (olanzapine, palonosetron, and DEX [OPD]). Chemotherapy-naïve patients age 18-80 years receiving single-day HEC were randomly assigned 1:1 to receive either the OPD regimen or the OPF regimen. The primary objective was to compare complete response (CR) rates for vomiting during the overall period (start of chemotherapy to 120 hours). Secondary objectives included CR for vomiting during the acute period (0-24 hours) and delayed period (24-120 hours), CR for nausea, and comparison of toxicities and patient-reported outcomes. RESULTS Three hundred forty-six patients received the study interventi