A 19-year-old patient with Williams syndrome and renal cystic dysplasia is presented. This young man has had gradual deterioration of renal function and recurrent, acute episodes of dehydration secondary to a concentrating defect. A review revealed several reports of renal dysfunction in patients with Williams syndrome, but this complication has not been sufficiently emphasized. We recommend that all patients with Williams syndrome undergo periodic ultrasound examination and renal function evaluation.