Background: Systemic treatment with immune combinations is the gold standard for metastatic renal cell carcinoma (mRCC) worldwide. The systemic immune-inflammation index (SII) is a prognostic marker for several types of malignant neoplasms, including mRCC, in the era of tyrosine kinase inhibitor (TKI) treatment. Data regarding the prognostic value of the SII in patients with mRCC treated with immunotherapy are scarce and controversial. METHODS: We retrospectively collected the data of patients with mRCC from 56 centers in 18 countries. SII (Platelet × Neutrophil/Lymphocyte count) was calculated prior to the first systemic treatment and cut-off was defined by a survival receiver operating characteristic (ROC) analysis. The primary objective of our retrospective study was to assess the outcomes of patients treated with first-line immunotherapy. RESULTS: Data from 1034 mRCC patients was collected and included in this analysis. The SII cut-off value was 1265. After a follow-up of 26.7 months, and the overall survival (OS) and progression-free survival (PFS) were 39.8 and 15.7 months, respectively. According to SII (low vs. high), patients with low-SII had longer OS (55.7 vs. 22.2 months, P < .001), better PFS (20.8 vs. 8.5 months, P < .001), and higher overall response rate (52 vs. 37%, P = .033)., Conclusion: A high SII is associated with poor oncological outcomes in patients with mRCC. SII could be an easily accessible prognostic indicator for use in clinical practice., Competing Interests: Disclosure Fernando Sabino M. Monteiro: Research support was provided by Janssen (Merck Sharp Dome). Honoraria from Janssen, Ipsen, Bristol Myers Squibb, and Merck Sharp Dome. Ownership: BIO, Brazilian Information Oncology; Ondrej Fiala: Received honoraria from Roche, Janssen, MSD, Pierre Fabre, GSK, and Pfizer for consultations and lectures unrelated to this project; Francesco Massari: Personal fees from Astellas, BMS, Janssen, Ipsen, MSD, and Pfizer; Jindrich Kopecky: Consulting fees from Bristol Myers Squibb, Ipsen, Novartis, MSD; payment or honoraria for lectures, presentations, speakers’ bureaus, manuscript writing, or educational events from Ipsen, Bristol-Myers Squibb, MSD, Pfizer, Merck, and Novartis; Jakub Kuchartz: Honoraria: Angelini, Astellas, Astra Zeneca, Bayer, Bristol Myers Squibb, IPSEN, Janssen, Merck MSD, Novartis, Pfizer, Research Funding: Novartis; Enrique Grande: Honoraria: Adacap, AMGEN, Angelini, Astellas, Astra Zeneca, Bayer, Blueprint, Bristol Myers Squibb, Caris Life Sciences, Celgene, Clovis-Oncology, Eisai, Eusa Pharma, Genetracer, Guardant Health, HRA-Pharma, IPSEN, ITM-Radiopharma, Janssen, Lexicon, Lilly, Merck KGaA, MSD, NanoString Technologies, Novartis, ONCODNA (Biosequence), Palex, Pharmamar, Pierre Fabre, Pfizer, Roche, Sanofi-Genzyme, Servier, Taiho, and Thermo Fisher Scientific. Funding: Pfizer, Astra Zeneca, Astellas, and Lexicon Pharmaceuticals; Maria T. Bourlon: Honoraria/advisory board: Astellas, Astra Zeneca, Bayer, Bristol Myers Squibb, Eisai, IPSEN, Janssen, Merck, MSD, Novartis, Pfizer, Roche. Funding: Pfizer; Javier Molina-Cerrillo: Consultant, advisory, or speaker roles for IPSEN, Roche, Pfizer, Sanofi, Janssen, Eisai, MSD, and BMS. JMC has received research grants from Pfizer, IPSEN, Janssen, and Roche; Tomas Buchler: Research support: AstraZeneca, Roche, Bristol Myers Squibb, Exelixis, Merck KGaA, MSD, and Novartis; consulting fees from Bristol Myers Squibb, Astellas, Janssen, and Sanofi/Aventis; payment or honoraria for lectures, presentations, speakers’ bureaus, manuscript writing, or educational events from Ipsen, Bristol-Myers Squibb, AstraZeneca, Roche, Servier, Accord, MSD, and Pfizer; Emmanuel Seront: Consultancy: MSD, BMS, Pfizer, Ipsen; Aristotelis Bamias: Honoraria, Advisory, Research support: Pfizer, Roche, MSD, BMS, and Ipsen; Camillo Porta: Honoraria: Angelini Pharma, Astra Zeneca, Bristol Myers Squibb, Eisai, Ipsen, MSD; Andre Poisl Fay: Honoraria: Pfizer, Astellas, BMS, Novartis, Roche, Astra-Zeneca, Janssen, MSD, Ipsen. Scientific Advisory Board: Janssen, Novartis, Roche, Ipsen, Pfizer, Bayer, MSD. Research Grant: CAPES - CNPq, BMS, Roche, Astra-Zeneca, MSD, Foundation Medicine, Ipsen. Ownership: BIO, Brazilian Information Oncology. All other authors do not have relationships to disclose. All authors except Ondrej Fiala are unrelated to this study., (Copyright © 2023 Elsevier Inc. All rights reserved.)