Prior studies suggest that the renal sinus permits early tumor spread in otherwise localized renal cell carcinoma (RCC) tumors. We hypothesized that renal sinus fat invasion may be unrecognized in pT1 patients who subsequently die from RCC. Between 1985 and 2002, we identified 577 patients who underwent radical nephrectomy for localized pT1 clear cell RCC as reviewed by a single urologic pathologist (J.C.C.). Among these patients, 49 died from RCC including 33 who had their original nephrectomy specimen stored in formalin. These specimens were then resectioned with thin cuts of the renal sinus and reviewed by the same pathologist. For comparison, 33 patients who did not die from RCC (controls) also had their original nephrectomy specimen resectioned. Among the 33 patients who died from seemingly localized RCC, 14 (42%) had previously unrecognized renal sinus fat invasion compared with 2 (6%) of the controls (P<0.001). In addition, 19 (58%) patients who died from RCC had renal sinus small vein (microscopic venous) invasion, a pathologic feature not currently incorporated into the TNM staging system for RCC. This feature was present in 7 (21%) of the controls (P=0.003). In total, 22 (67%) patients who died from RCC had unrecognized renal sinus fat or small vein invasion compared with 7 (21%) of the controls (P<0.001). We conclude that renal sinus fat invasion is an important adverse pathologic feature that is clearly underreported in the literature. Appropriate assessment of nephrectomy specimens should include proper sampling of the renal sinus even for seemingly localized tumors.