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BACKGROUND Selection of patients for upfront cytoreductive nephrectomy (CN) in metastatic renal cell carcinoma (mRCC) has to be improved. OBJECTIVE To evaluate a new scoring system for the prediction of overall mortality (OM) in mRCC patients undergoing CN. DESIGN, SETTING, AND PARTICIPANTS We identified a total of 519 patients with synchronous mRCC undergoing CN between 2005 and 2019 from a multi-institutional registry (Registry for Metastatic RCC [REMARCC]). OUTCOME MEASUREMENTS AND STATISTICAL ANALYSIS Cox proportional hazard regression was used to test the main predictors of OM. Restricted mean survival time was estimated as a measure of the average overall survival time up to 36 mo of follow-up. The concordance index (C-index) was used to determine the model's discrimination. Decision curve analyses were used to compare the net benefit from the REMARCC model with International mRCC Database Consortium (IMDC) or Memorial Sloan Kettering Cancer Center (MSKCC) risk scores. RESULTS AND LIMITATIONS The median follow-up period was 18 mo (interquartile range: 5.9-39.7). Our models showed lower mortality rates in obese patients (p = 0.007). Higher OM rates were recorded in those with bone (p = 0.010), liver (p = 0.002), and lung metastases (p < 0.001). Those with poor performance status (<80%) and those with more than three metastases had also higher OM rates (p = 0.026 and 0.040, respectively). The C-index of the REMARCC model was higher than that of the MSKCC and IMDC models (66.4% vs 60.4% vs 60.3%). After stratification, 113 (22.0%) patients were classified to have a favorable (no risk factors), 202 (39.5%) an intermediate (one or two risk factors), and 197 (38.5%) a poor (more than two risk factors) prognosis. Moreover, 72 (17.2%) and 51 (13.9%) patients classified as having an intermediate and a poor prognosis according to MSKCC and IMDC categories, respectively, would be reclassified as having a good prognosis according to the REMARCC score. CONCLUSIONS Our findings confirm the relevance of tumor and patient features for the risk stratification of mRCC patients and clinical decision-making regarding CN. Further prospective external validations are required for the scoring system proposed herein. PATIENT SUMMARY Current stratification systems for selecting patients for kidney removal when metastatic disease is shown are controversial. We suggest a system that includes tumor and patient features besides the systems already in use, which are based on blood tests.