A 10-year-old boy underwent a computed tomography (CT) scan for left flank pain following a fall. Imaging demonstrated a 5 cm left upper pole renal mass. Partial nephrectomy revealed metanephric adenofibroma, a benign stromal-epithelial tumor thought to represent a hyperdifferentiated, mature form of Wilms' tumor. We briefly discuss the histopathology and management of this rare tumor.