Incidental carcinoma of kidney is an increasingly common pathology as a result of a wider use of high resolution diagnostic methods. The special characteristics of these tumors, usually small and in a low grade or stage, make conservative surgery to be considered as a valid therapeutical option. An incidental carcinoma of kidney with tumoral thrombosis of the renal vein undetected in the pre-surgical study, circumstance which allows to emphasize the malignant potential of these tumors is presented here. Based on this, we believe that a meticulous evaluation of the renal vein condition should be made, in patients considered as possible candidates for conservative surgery. In this sense, selective renal phebography and magnetic resonance could be considered explorative means that play an important role in the introduction of renal preservation surgery, if future studies show that its use allows an early detection of the tumoral condition of renal vein.