Background: Evidence-based antiemetic guidelines offer predominantly consistent recommendations for chemotherapy-induced nausea and vomiting (CINV) prophylaxis. However, studies suggest that adherence to these recommendations is suboptimal. We explored inconsistencies between clinical practice and guideline-recommended treatment with a registry evaluating the effect of guideline-consistent CINV prophylaxis (GCCP) on patient outcomes., Patients and Methods: This was a prospective, non-interventional, multicentre study. The primary objective was to assess the overall (Days 1-5) complete response (CR: no emesis/no rescue use) rates in patients who received GCCP or guideline-inconsistent CINV prophylaxis (GICP) using diaries for 5 days following chemotherapy. Cycle 1 results are presented in patients who received either (1) anthracycline/cyclophosphamide (AC) highly emetogenic chemotherapy (HEC), non-AC HEC or carboplatin, with GCCP for all these groups consisting of prophylaxis with an NK 1 receptor antagonist (RA), 5-HT 3 RA and dexamethasone prior to chemotherapy or (2) moderately emetogenic chemotherapy (MEC), with GCCP consisting of a 5-HT 3 RA and dexamethasone prior to chemotherapy as per MASCC/ESMO 2016 guidelines, in place at the time of the study., Results: 1,089 patients were part of the cycle 1 efficacy evaluation. Overall GCCP was 23%. CR rates were significantly higher (P < 0.05) in patients receiving GCCP (62.2%) versus GICP (52.6%) in the overall population, as well as in the subsets of patients receiving AC/non-AC HEC (60.2% versus 47.8%), MEC (73.8% versus 57.8%) and in those non-naïve to the chemotherapy received (65.9% versus 53.8%). No impact on daily living due to CINV (FLIE assessment) was observed in 43.4% patients receiving GCCP versus 28.5% GICP (P < 0.001)., Conclusion: Consistent with prior studies, GCCP was very low; a significant benefit of almost 10% improved prevention of CINV was observed with GCCP. As per MASCC/ESMO guidelines, such an absolute difference should be practice changing. Comprehensive multifaceted strategies are needed to achieve better adherence to antiemetic guidelines., Competing Interests: Conflict of interest statement The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: M. Aapro: Consultant for: Helsinn, Mundipharma, Tesaro; Speakers Bureau of: Helsinn, Mundipharma, Tesaro. Z. Caprariu: Grant/research support from: Astellas, Astra Zeneca, Merck KGaA, MSD, and Pfizer; Speakers Bureau of: Roche, Eli Lilly, KRKA, G.L. Pharma - Romania. R.O. Curca: Grant/Research Support from: Abbvie, Astra Zeneca, Merck, Roche/Genentech, Helsinn; Speakers Bureau of: Abbvie, Accord, Amgen, Angelini, Astellas, AstraZeneca, Bayer, BMS, Eli Lilly, Ipsen, Johnson&Johnson, Merck KGaA, MSD, Novartis, Sandoz, Sanofi, Roche. L.Gales: Grant/Research Support from: Abbvie, Accord, Amgen, Angelini, Astellas, AstraZeneca, Bayer, BMS, Eli Lilly, Ipsen, Johnson&Johnson, Merck KGaA, MSD, Novartis, Sandoz, Sanofi, Roche, Helsinn, Tesaro; Consultant for: Abbvie, Accord, Amgen, Angelini, Astellas, AstraZeneca, Bayer, BMS, Eli Lilly, Ipsen, Johnson&Johnson, Merck KGaA, MSD, Novartis, Sandoz, Sanofi, Roche, Helsinn, Tesaro; Speakers Bureau of: Abbvie, Accord, Amgen, Angelini, Astellas, AstraZeneca, Bayer, BMS, Eli Lilly, Ipsen, Johnson&Johnson, Merck KGaA, MSD, Novartis, Sandoz, Sanofi, Roche, Helsinn, Tesaro. R. Kellnerová: Employee: Angelini Pharma Česká republika s.r.o. M. Malejčíková: Consultant for: Angelini Pharma Slovenská republika s.r.o., Amgen, Eli Lilly, Mundipharma, Novartis, Pfizer, Roche, Sandoz, Teva. M. Marinca: Consultant for Angelini Pharma, Roche; Speaker's bureau for Angelini Pharma, Roche; Grant/Research funding from Roche. E. Petru: Honoraria Received for Lectures: MSD, Angelini Pharma. M. Rubach: Consultant for: Angelini Pharma Poland. G.G. Steger: Consultant for: Amgen, TEVA, Novartis, Roche. The remaining authors disclose no conflicts., (Copyright © 2022 The Authors. Published by Elsevier Ltd.. All rights reserved.)