A prospective angiographic study of parasitic arterial supply in unselected cases of renal cell carcinoma was correlated, following nephrectomy, with pathologic evidence of transcapsular tumor spread. Parasitic arterial supply was shown in 9 of the 10 cases in which transcapsular tumor extension had occurred, but it was also demonstrated in 7 of the 11 cases in which tumor was entirely confined within the renal capsule. Selective angiographic study of parasitic supply was not found to be a reliable method for the recognition of early transcapsular tumor extension. The common sources of parasitic supply to renal tumors derive from the perirenal arterial network, which communicates with the intrarenal circulation through capsular perforating branches. Less commonly it is obtained from intestinal vessels when these structures are in close relationship. Profuse panasitic supply from inferior mesenteric arterial branches is reported in a case where the renal carcinoma was entirely confined within the renal capsule. It is concluded that identification of parasitic arterial supply to renalcarcinomas should not, in itself, be regarded as evidence of extrarenal extension of tumor or inoperability.