Partial nephrectomy is the standard treatment for T1a and select T1b tumors. Tumor location is very important because it may predict surgical complications and malignant potential. The aim of this study was to investigate the association between adipose tissue, especially around the kidney, and the growth pattern of RCC. We retrospectively reviewed computed tomography scans of 153 patients with stage 1 renal cell carcinoma who underwent radical or partial nephrectomy at our hospital between January 2013 and July 2016. We calculated visceral/subcutaneous/perirenal fat volumes using SYNAPSE VINCENT®. In 60 patients, perirenal fat was immunohistochemically stained for leptin, adiponectin, COX-2 and UCP1, and the association with outward tumor protrusion was evaluated. Among the 153 cases, 88 had confirmed outward expansion (57.5%), 110 were classed as pT1a (52 and 58 with outer and inner expansion, respectively), and 43 were classed as pT1b (36 and seven with outer and inner expansion, respectively; pT1a vs. pT1b, P < 0.0001). A multivariate logistic regression model showed a trend towards significance in pT1b (vs. pT1a; odds ratio [OR] 6.033; 95% confidence interval [CI] = 2.409– 15.108; P = 0.0001) and perirenal fat percentage >1.0 (vs. ≤1.0; [OR] 2.596; 95% CI = 1.205– 5.591; P = 0.014) as independent predictors for outer protrusion. Immunohistochemical staining was positive for UCP1 expression in 31 out of 41 outgrowth types (75.6%) and in all 19 endogenous types (100%; P = 0.003). Renal cell carcinoma with thick perirenal fat correlates with an increased likelihood of developing outward tumor protrusion; therefore, fat distribution may affect the development of renal cell carcinoma.