Background: Sarcomatoid renal cell carcinoma (sRCC) is a histologic feature that denotes an aggressive variant of kidney cancer and worse overall outcomes. Our aim was to determine if the percentage of sarcomatoid differentiation (% Sarc) could be used for prognostic risk stratification. Methods: We performed a retrospective analysis of patients who underwent surgery at our center and found to have sRCC. A single genitourinary pathologist reviewed each specimen for %Sarc and other pathologic variables of interest. %Sarc was analyzed as a continuous variable and as a binary variable using cut-points of 5%, 10%, and 25%. Potential prognostic factors associated with overall survival (OS) were determined using the Cox regression model. OS curves were generated with Kaplan-Meier methods and survival differences compared using the log-rank test. Results: Between 1998 and 2012, 1,307 consecutive cases of RCC were identified, of which 59 patients were confirmed to have sRCC (4.5%). As a continuous variable %Sarc was associated with OS (p=0.023). Predictors of survival on multivariable analysis included pT stage, tumor size, cM stage and % Sarc at the 25% binary level. OS was most dependent on the presence or absence of metastatic disease (4 months vs. 21.2 months, p=0.001). However, in a subgroup analysis of cM0 patients with locally advanced (≥ pT3) tumors, OS was significantly diminished in patients with >25% Sarc compared to ≤25% Sarc (p=0.045). OS relative to %Sarc was no different in subgroup analyses of patients with early stage disease (pT1-T2, M0) or in patients with clinical metastatic disease. Conclusions: Patients with sRCC have a poor overall outcome as evidenced by high rates of recurrence and death. Patients without clinical metastatic disease and >25% Sarc have a higher risk of relapse and worse OS. More effective understanding of the biological basis for the aggressive behavior of sarcomatoid RCC is needed, and nomograms to predict recurrence or survival following nephrectomy could incorporate this pathologic feature for added risk stratification.