Tumorectomy was performed in a patient with hypervascular renal carcinoma on a solitary kidney. Prior to surgery, this patient had mild renal impairement (plasma creatinine 195 mumol/l, creatinine clearance 48 ml/min), but it was noted that 45 days after surgery his renal function had improved (plasma creatinine 142 mumol/l, creatinine clearance 63 ml/min). The hypothesis of a prerenal model of renal impairment is put forward, the probable physiopathological mechanism being a diversion of blood flow from the healthy kidney to the hypervascularized tumoral kidney.