Eighty-one cases of multicystic dysplastic kidney (MCDK) in children were diagnosed over the past 11 years at the authors' institution: 25 children had their kidneys surgically removed, eight with bilateral total involvement died, and 48 underwent serial follow-up ultrasonography (US) of their kidneys. Follow-up included 193 serial ultrasound (US) studies (mean, four per patient) for a total of 1,468 months (mean, 30.5 months). In the 48 patients followed up, 32 (67%) kidneys showed a decrease in size, nine (19%) showed no change, and five (10%) increased in size, and in two (4%), a change in size could not be determined. In seven of the 48 (15%) children, the MCDKs decreased in size, and, at follow-up US, no renal tissue could be found. In those patients in whom MCDKs decreased in size. Serial US characteristics changed from predominantly an enlarged cystic structure to a small dysplastic or absent kidney. Two of the five kidneys that increased in size were surgically removed, and MCDK was pathologically confirmed. A nonsurgical approach to the treatment of patients with MCDK is supported by this study.