Radiographic and pathologic findings of surgically proved 2 cases of multicystic dysplastic kidney arepresented with great emphasis on the ultrasonographic features. The first case was 2 week old male and the secondcase was 18 month old female who were admitted with the same complaint of nonsymptomatic abdominal mass. IVPfindings were unilateral non-visualized kidney suggestive of renal origin mass, but in the second case due tofaint rim sign, erroneous diagnosis of the hydronephrosis were given. By the ultrasonography, preoperativecorrective diagnosis of multicystic dysplastic kidney was possible in both cases. Characteristic findings anddifferential points with other mass lesions were as follows. 1. Multiple round cysts, separated each other withdistinct interface. 2. Absence of renal parencymal and sinus echo. 3. No distinct outer cortical margin.Nephrectomy was performed in both cases and the specimen showed findings of multicystic kidney including theureteral atresia.