Beckwith Wiedemann Syndrome (BWS) is a congenital overgrowth syndrome associated with an increased risk of Wilms tumor (WT). Based on a high rate of WT, periodic abdominal sonography exams were recommended. We investigated whether nephromegaly was a risk factor for the development of Wilms tumor. The radiographic finding of 49 patients with BWS were reviewed. Renal lengths were measured from available sonographic, MR or CT imaging. Nephromegaly was defined as at least one kidney greater than the 95th percentile for length. The patients were divided into two categories: Those with WT (n=15) and those without WT (n=25). Patients with non malignant renal abnormalities were excluded from analysis (n=9). A total of 15 patients had WT. All 15 of these patients had nephromegaly (three unilateral, 12 bilateral nephromegaly). Nephromegaly was detected by surveillance ultrasound in 10 of these 15 patients. Six of 25 patients without WT demonstrated nephromegaly. The odds ration for Wilms tumor with nephromegaly was 48.0 (95%CI=6.0-350.0). No patient with WT had a normal sized kidney on any scan. Thus, the negative predictive value for a normal kidney and subsequent WT was 100%. BWS patients with nephromegaly are at greater risk for the development of WT; more frequent surveillance with ultrasound exams may be warranted in this subgroup of patients.