Purpose: MCDK is regarded as a common cause of abdominal masses in children. And the presentation of the MCDK is usually a unilateral flank mass in the a newborn. Bialteral disease results in either fetal demise or necessity fer renal replacement therapy at birth. This study is designed to assess the clinical features and natural history of the unilateral multicystic dysplastic kidney. Patients and Methods: From January 1987 to January 2000 data were obtained retrospectively on 57 patients (28 boys and 29 girls, age ranged 1day-11years) who had a diagnosis of multicystic dysplastic kidney. The diagnosis of multicystic dysplastic kidney was confirmed by a combination of ultrasonography and radionuclide scan. Voiding cystourethrogram study in 31 patients were done to determine the condition of the contalateral kidney. Restllts: 84%
84%
of the patients were diagnosed before birth by antenatal ultrasonography Clinical manifestations of children with postnatal diagnoses were palpable abdominal mass( 3.5%
3.5%
), abdominal distension( 17%
17%
), and incidental( 10.5%
10.5%
). The abnormalities in contralateral kidney were hydronephrosis( 21%
21%
), compensatory hypertrophy( 12%
12%
), simple cyst( 2%
2%
), bifid pelvis( 2%
2%
). Surgical management was performed in 20 patients( 35%
35%
) due to recurrent infection, for diagnostic purpose to differentiate from malignancy and abdominal distention. Follow-up in the remaining 37 patients continued (mean 18 months) and results of sonogram findings were involution change in 23 patients( 40%
40%
) and no interval changes in 13 patient( 23%
23%
). Conclusions : The apparent tendency to regression of the dysplastic kidney and no difference in the number of complications justify a conservative management rather than operative intervention except in associated severe complications such as urinary tract infection or rupture of cysts.