PURPOSE: To characterize the ultrasonographic findings of multicystic dysplastic kidney on prenatal ultrasonography (US) with a special emphasis on the distribution of cysts. MATERIALS AND METHODS: From January 1998 to March 2003, medical records of sixty two subjects with multicystic dysplastic kidney diagnosed on prenatal US examination were retrospectively reviewed, and forty three patients confirmed either by pathology or postnatal follow-up US were selected for this study. US assessment included the time of diagnosis, laterality, size of the multicystic dysplastic and contralateral normal kidneys, distribution of cysts and associated anomalies. The distribution of cysts was categorized as subcapsular and random distribution, and interobserver agreement was determined using the cross table analysis. The largest multicystic and contralateral normal longitudinal diameters were measured, and the data were plotted on the normal reference chart. RESULTS: Multicystic dysplastic kidney was left sided in 55.8%, right sided in 34.8% and bilateral in 9.3%. Subcapsular distribution of cysts was observed in 68.2% (n=15) for radiologist 1 while 59.1% (n=13) for radiologist 2, showing an excellent interobserver agreement (k=0.697). The longitudinal diameter of the multicystic dysplastic kidney was above 95 percentile in 68%. Meanwhile, the diameter of the contralateral normal kidney was more commonly normal, 70%. Fetal karyotyping was done in 18 cases including 2 cases with associated major anomalies, but karyotyping was all normal. CONCLUSION: On prenatal US, subcapsular distribution of cysts in multicystic dysplastic kidney is more common than random distribution. This characteristic distribution of cysts may be helpful in the prenatal diagnosis of multicystic dysplastic kidney.