The 2002 TNM formulation defines a pT3b tumor as one that 'extends into the renal vein or its segmental (muscle containing) branches.' This definition elicits uncertainty when veins with little muscle are involved or the relationship to the main renal vein is unknown. The diameter and medial thickness of 10 normal renal venous systems were studied and compared to sinus veins involved in 54 pT3b clear cell renal cell carcinomas (CC). All tumors were grossly examined and sampled for histology by the author. An immunoperoxidase cocktail containing CD 31 and actin, Masson trichrome and elastic stains were employed to aid identification of intravenous tumor. The venous dissections showed variable numbers of primary and secondary divisions with substantial overlap in diameter and medial thickness. The medial thickness decreased with each proximal division and ranged from being nonexistent to being thick. Study of the 54 pT3b CC revealed that the initial phase of extrarenal extension involved large caliber veins draining the primary tumor. With extensive venous involvement, tumor invaded through the vein wall into sinus fat or demonstrated retrograde venous extension into adjacent cortex. Correlation between gross and histology revealed that most nodules of tumor within the sinus fat contained evidence of pre-existing veins. The following observations were made: (1) the diameter of a sinus vein or the quantity of muscle is a poor indicator of vein segment or relationship to the main renal vein; therefore, the wording used to define pT3b should be clarified; (2) extrarenal spread in CC begins with intravenous extension whereas sinus fat invasion is usually secondary; (3) retrograde venous extension occurs in cases with massive renal vein involvement; and (4) nodules within the sinus fat usually represent venous involvement.