PURPOSE: Conservative management of multicystic dysplastic kidney(MCDK) without nephrectomy has recently been advocated. The purpose of this study was to determine the clinical course of conservatively managed MCDK and to find out possible predictive factors for involution of MCDK by ultrasonography(US). METHODS: A retrospective analysis was made on 46 patients(26 boys and 20 girls) in whom MCDK was detected and had been traced by US between Dec. 1993 and Aug. 2005 at Severance Hospital. RESULTS: Median follow-up time was 30 months(range 2-102 months). All patients underwent radionuclide scans and voiding cystourethrograms. The serial follow-up US showed complete involution in 11(24%), partial involution in 19(41%), and no interval change or increased in cyst size in 13(28%) patients. Nephrectomy was done in 3 patients(7%) due to relapsing urinary tract infection(UTI) and severe abdominal distension. The mean age of complete involution of MCDK was 37 months(range 12-84 months). Episodes of UTI were present in 17 patients(37%) and additional genitourinary(GU) abnormalities were found in 22 patients(44%). Hypertension and renal insufficiency was complicated in one patient. No child developed malignant tumor. Univariate analysis showed that five variables were associated with complete involution of the MCDK; gender, site, UTI episode, additional GU abnormalities, and renal length on initial US. After adjusting using the Pearson model, the presence of additional GU abnormalities was exclusively associated with complete involution among the 5 variables(P=0.034). CONCLUSION: In our review of 46 cases of MCDK, non-surgical approach for patients with MCDK was advisable and we could predict poor prognosis when MCDK is associated with other GU anomalies.