Presence of nephrogenic rests (nephroblastomatosis) in resected specimen indicates an increased risk of developing Wilm's tumor in contralateral kidney. Nephrohlastomatosis also k/a "nephrogenic rests" or nodular renal blastema, refer to the abnormal persistence of embryonal renal tissue. These are supposed to be precursor lesions of Wilm's tumor. These are seen in renal paraenchyma adjacent to tumors in approx. 25% to 40% of unilateral and 100% of bilateral tumors. Presence of nephrogenic rests in resected specimen indicates an increased risk of developing Wilm's tumor in contralateral kidney. So these patients will require frequent and regular radiological surveillance with CT scan for many years. Ref : Nelson Pediatrics 18/e p2140-2143