OBJECTIVE. The objective of this retrospective review was to determine the incidence and spectrum of nonmalignant renal disease in patients with Beckwith-Wiedemann syndrome. MATERIALS AND METHODS. Patient records were obtained from the Beckwith-Wiedemann Registry of the National Cancer Institute. Imaging findings and medical records of 152 neonates, infants, children, and adults with Beckwith-Wiedemann syndrome (age range, 1 day to 30 years old; median age, 1 year 3 months old) were retrospectively reviewed by three radiologists. Available pathologic material also was reviewed. RESULTS. Thirty-eight (25%) of 152 patients with Beckwith-Wiedemann syndrome had 45 nonmalignant renal abnormalities, including medullary renal cysts (n = 19, 13%), caliceal diverticula (n = 2, 1%), hydronephrosis (n = 18, 12%), and nephrolithiasis (n = 6, 4%). Thirty-three (87%) of the 38 patients with nonmalignant renal disease were asymptomatic. Clinical manifestations of the remaining five patients included urinary tract infections (n = 4) and flank pain due to obstructive stone disease (n = 1). Nonmalignant renal disease was mistaken for Wilms' tumor in two patients, resulting in unnecessary nephrectomies. Seven children (18%) had Wilms' tumor and nonmalignant renal disease. CONCLUSION. Nonmalignant renal abnormalities occur in approximately 25% of patients with Beckwith-Wiedemann syndrome but are generally asymptomatic. Nonmalignant renal abnormalities should be considered in the differential diagnosis of a mass revealed during screening sonography of a patient with Beckwith-Wiedemann syndrome to avoid unnecessary surgery.