Radiographic and pahtologic findings of surgically proved 2 cases of multicystic dysplastic kidney are presented with great emphasis on the ultrasonographic features. The first case was 2 week old male and the second case was 18 month old female who were admitted with the same complaint of nonsymptomatic abdominal mass. IVP findings were unilateral non-visualized kidney suggestive of renal origin mass, but in the second case due to faint rim sign, erroneous diagnosis of the hydronephrosis were given. By the ultrasonography, preoperative corrective diagnosis of multicysitc dysplastic kidney was possible in both cases. Characteristic findings and differential points with other mass lesions were as follows. 1. Multiple round cysts, separated each other with distinct interface. 2. Absence of renal parenchymal and sinus echo. 3. no distinct outer cortical margin. Nephrectomy was performed in both cases and the specimen showed findings of multicystic kidney including the ureteral atresia.