Renal oncocytoma followed for eighteen years without resection

The natural history of unresected renal oncocytomas is unknown. We report a renal oncocytoma which was mistakenly diagnosed as benign renal cyst and followed up for eighteen years. We are unable to find other oncocytomas without resection which were followed up. The apparent lack of change in size over eighteen years supports the concept of slow growth. Malignant histologic characteristics confirm the potential for malignant degeneration in what is generally considered to be a benign lesion.