Background: Baseline sarcopenia and postoperative changes in muscle mass are independently associated with overall survival in patients with metastatic renal cell carcinoma (mRCC) undergoing cytoreductive nephrectomy (CN). Here we examine the relationship between preoperative (baseline) and postoperative changes in muscle quantity with survival outcomes following CN as determined by linear segmentation, a fast and clinic-friendly tool. Methods: Our nephrectomy database was reviewed for patients with clear cell, papillary, or chromophobe mRCC who underwent CN. Linear segmentation of bilateral psoas/paraspinal muscles was completed for baseline imaging within 60 days of surgery and imaging up to 1 year postoperatively. ANOVA for numerical and chi-square for categorical variables were used to test for differences according to change in linear muscle index (LMI, cm2/m2). Multivariable models estimated COX hazard ratios for cancer-specific survival (CSS) and overall survival (OS). Kaplan Meier curves estimated CSS and OS. Results: From 2004-2020, 190 patients were identified 48 stable LMI (25.3%; <5% change [0Δ]), 54 increase LMI (28.4%; +5% change [+Δ]), and 88 decrease LMI (46.3%; -5% change [-Δ]). Median time from baseline imaging to surgery was 18 days, while time from surgery to postoperative imaging was 119 days. Patients with +Δ had lower baseline LMI than -Δ or 0Δ (28.5 vs. 32.4 vs.32.5 cm2/m2; p=0.003). 0Δ LMI had lower rates of pN1 disease than other groups (27.1% [0Δ] vs. 42.6% [+Δ] vs. 45.5% [-Δ]; p=0.019). No other differences in pathology were noted. Median CSS and OS were highest among patients with 0Δ LMI (CSS: not reached [0Δ] vs. 61.9 [+Δ] vs. 37.4 [-Δ] months; p=0.0018 || OS: 67.2 [0Δ] vs. 48.5 [+Δ] vs. 26.4 [-Δ] months; p=0.0007). Median follow-up was 56 months for survivors. The table lists factors associated with increased risk of cancer-specific mortality. Conclusions: Change in muscle mass after CN, as measured by the linear muscle segmentation technique, is independently associated with OS and CSS in patients following CN. Of note, lack of change demonstrated greatest survival, potentially secondary to high baseline muscle mass.