Arteriovenous malformations are important but uncommon differential diagnoses for space occupying renal lesions with unknown genesis. In the following case report, a 41-year-old patient complained about continuing right flank pain. Computer tomography indicated a possible renal cell carcinoma and the patient was admitted to our clinic. Further diagnostics, including contrast medium-enhanced bolus CT, colour coded duplex sonography, renovasography and MR-angiography, revealed a large arteriovenous fistula of the entire right kidney. Because of the size and aneurysmatic appearance of this fistula, we performed a nephrectomy. Since this operation the patient has been free of complaints.