Nephrogenic adenofibroma is a recently described subset of tumors from the National Wilms Tumor Study pathology center.' We report a case of this lesion found at partial nephrectomy for an indeterminate renal mass. CASE REPORT A 36-year-old man presented with left flank discomfort and microscopic hematuria. Physical examination was normal, blood pressure was 152/68 mm. Hg and hemoglobin was 15.3 gm./dl. (normal 14 to 18). Excretory urography showed no obstruction but it suggested a left lower pole mass. Ultrasonography and computerized tomography (CT) showed a 2 cm. right lower pole mass that was hyperechoic on ultrasound and hyperdense on noncontrast CT (fig. 1). Ultrasound confirmed the cystic nature of the left lower pole mass seen on excretory urography and CT. Cystoscopy was normal. Right lower pole partial nephrectomy was performed. Intraoperatively frozen sections suggested a neoplasm of uncertain type with a positive resection margin after wedge excision. Completion of lower pole nephrectomy yielded a negative margin. Followup CT at 6 and 12 months showed no evidence of disease. Microscopic hematuria has resolved. The specimen consisted of a tan nodular 2 X 1.7 X 1.2 cm. mass. Microscopically the tumor was nonencapsulated but circumscribed. It comprised multifocal nests of tubulopapillary and papillary configurations of benign appearing embryonal epithelium surrounded by stroma composed of benign appearing fibroblast-like cells (fig. 2). Numerous psammoma bodies were noted. Immature cells were cuboidal or columnar with round to oval nuclei containing peppery chromatin and inconspicuous nucleoli surrounded by scant amphophilic cytoplasm. Cell borders were indistinct, no mitotic figures were identified and nuclei overlapped. The tumor was markedly vascularized and many vascular spaces were lined with endothelial cells. Some larger blood vessels had thick walls and lacked muscular organization. Neither necrosis nor inflammatory exudate was noted. There was no evidence of collecting duct carcinoma or cysts in the tumor. Immunohistochemistv study was not performed.