Nephroblastomatosis, a congenital lesion of infants and children, is marked by the persistence of metanephric blastema in the kidney. Computed tomography (CT) has previously been reported to be the best way to image these precancerous lesions. The authors discuss the radiologic and surgical implications of a case of macroscopically visible nephroblastomatosis demonstrated at histologic examination but not detected with CT in the contralateral kidney of a patient with Wilms tumor.