Background: Chemotherapy-induced nausea and vomiting (CINV) is a common side effect of highly emetogenic chemotherapy (HEC). Current guidelines recommend the use of dexamethasone (DEX) for prophylaxis, but the effectiveness and safety of a DEX-free regimen for CINV prophylaxis in HEC is not known. Methods: This was an investigator-initiated, double-blind, phase 3 RCT designed to show the non-inferiority of a DEX-free regimen (olanzapine, palonosetron, and fosaprepitant; OPF) compared to the NCCN recommended DEX-containing regimen (olanzapine, palonosetron, and dexamethasone; OPD). Chemotherapy-naive patients aged 18-80 years receiving single-day HEC were randomized 1:1 to receive either the OPD regimen (olanzapine 5 mg once a day on days 1-4, palonosetron 0.25 mg on day 1, and DEX 12 mg on day 1) or the OPF regimen (olanzapine and palonosetron as above, and fosaprepitant 150 mg on day 1). The primary endpoint was to compare complete response (CR) rates for vomiting (no vomiting and no rescue medication) during the overall phase (start of chemotherapy to 120 h). The secondary endpoints were CR for nausea (ESAS), toxicities (CTCAE), and patient reported outcome measures (PROM) using MDASI. Assuming CR for vomiting for the overall phase of 77% (power = 90%, 2-sided alpha = 5%, and non-inferiority margin [NI] = 15%), 175 patients in each arm were required (including 5% dropout). Results: Between May 2022-Dec 2022, 350 patients were randomized and 346 received the study interventions (OPD arm n = 174 and OPF arm n = 172). The most common chemotherapy used was adriamycin/cyclophosphamide (66%) followed by cisplatin (27%). CR rates for vomiting for the overall phase were 48.8% in the OPD arm and 79.6% in the OPF arm (P < 0.001), with an absolute difference of -30.8% and 2-sided 95% CI = (-40.31, -21.24) which was within the pre-defined NI margin of 15% and also showed superiority. The DEX-free arm also had significantly higher CR rates for nausea and total control (vomiting and nausea combined) (Table). There were no significant differences in any grade CTCAE toxicities. PROM for fatigue (P = 0.009) and drowsiness (P = 0.002) were more in the OPF arm in the acute phase, and insomnia (P < 0.001) in the OPD arm in the delayed phase. Conclusions: This study shows that a DEX-free OPF regimen is superior to the DEX-based OPD regimen and should be considered a standard option for CINV prophylaxis for HEC. Clinical trial information: CTRI/2022/03/040906.