A 14-year-old boy presented with painless gross hematuria after a blunt trauma to his right lumbar region. Abdominal ultrasonography and abdominal computerized tomography revealed that the right kidney was hydronephrotic; contrast material delineated a hematoma inside the kidney. The patient underwent a right simple nephrectomy and after histopathological examination was found to have a renomedullary interstitial cell tumor of the right kidney. This tumor is common in patients older than 50 years, whereas our patient was very young. The diagnosis of this small tumor is not possible by conventional radiological techniques and histopathological examination is mandatory for its diagnosis.