Kidneys and urinary tract were examined systematically by ultrasonography in 130 patients with Williams-Beuren syndrome (59 females, median age 5.5 years; 71 males, median age 6.4 years). In addition, serum creatinine was determined and an analysis was performed. Creatinine clearance was available in 79 patients. Renal angiographic examinations were done in 18 patients, 8 of whom had renal artery narrowing (44%). The incidence of renal anomalies in Williams-Beuren syndrome was 17.7% vs. around 1.5% in the normal population (P < 0.0003). The spectrum of these anomalies ranged from minor anomalies such as bladder diverticula to more severe malformations such as renal aplasia or hypoplasia (in 5 of 130 patients). In nine patients a duplicated kidney was found. A decreased creatinine clearance (two patients), recurrent symptomatic urinary tract infections (four patients), and hypertension were uncommon. Nephrocalcinosis was not found in our patients. Our data demonstrate that the risk of a structural abnormality of the kidneys and the urinary tract is increased 12- to 36-fold in Williams-Beuren syndrome compared to the normal population. Ultrasound screening of the renal system should be part of the first evaluation of WBS patients.