['To assess whether extreme obesity (body mass index [BMI] ≥ 40 kg/m(2) ) is associated with peri-operative outcomes, overall survival (OS), cancer-specific survival (CSS), or recurrence-free survival (RFS) after surgical treatment for renal cell carcinoma (RCC).', 'After institutional review board approval, we used an institutional database to identify patients treated surgically between January 2000 and December 2014 with a pathological diagnosis of RCC. Comprehensive clinical and pathological data were reviewed. Kaplan-Meier analyses were used to estimate OS, RFS and CSS. Univariate and multivariate Cox proportional hazards analysis was used to evaluate associations with OS, CSS and RFS in patients with extreme obesity, among other known predictive variables.', 'In all, 100 patients (11.9%) with a BMI ≥ 40 kg/m(2) and 743 patients (88.1%) with a BMI < 40 kg/m(2) who were treated surgically for RCC were identified. Morbid obesity was not associated with an increased risk of blood transfusion (odds ratio [OR] 1, 95% confidence interval [CI] 0.587-1.70; P = 1.0). The median (interquartile range) length of hospital stay (LOS) was 4 (3-6) days. Morbid obesity was not associated with longer LOS (P = 0.26) or 30-day hospital readmission rates (P = 1.0). Major complications (Clavien ≥ 3a) were recorded in 67 patients (7.95%). BMI ≥ 40 kg/m(2) was not a predictor of major complications (OR 0.58, 95% CI 0.227-1.47; P = 0.251) or 90-day mortality (P = 0.4067). BMI ≥ 40 kg/m(2) was not associated with worse OS (P = 0.7), CSS (P = 0.2) or RFS (P = 0.5). BMI ≥ 35 kg/m(2) was also not associated with worse OS, CSS or RFS (P = 0.3, 0.1, 0.5, respectively). The 5-year OS rate was 68.9% for the entire cohort, including 69 and 70% for patients with BMI < 40 kg/m(2) and BMI ≥ 40 kg/m(2) , respectively (P = 0.69). The 5-year CSS was 79.5% for the entire cohort, including 78.4 and 87.9% (P = 0.16) for patients with BMI < 40 kg/m(2) and BMI ≥ 40 kg/m(2) , respectively. The 5-year RFS rates for BMI < 40 kg/m(2) and BMI ≥ 40 kg/m(2) were 84.1 and 90.6%, respectively (P = 0.48).'] Does extreme obesity predict poor cancer outcomes after surgery for renal cell cancer?