Multilocular renal cyst, a well-encapsulated benign lesion consisting of multiple noncommunicating cysts, is an uncommon entity of uncertain nature. A spectrum of histologic findings and multiple theories of pathogenesis have resulted in numerous names for the lesion, including multilocular cyst, benign cystic nephroma, cystic hamartoma, cystic lymphangioma, cystic Wilms tumor, and Perlmann tumor. We believe these names, and others, refer to the same entity, namely multilocular cyst. Radiographically a multicystic renal mass which is often calcified and which may partially protrude into the renal pelvis is seen. Although angiography cannot definitively exclude a malignant tumor, a distinctive sonographic appearance-multiple cystic masses separated by highly echogenic septa-is highly suggestive of multilocular renal cyst. Inclusion of this lesion in the preoperative differential diagnosis of a solitary and presumed malignant renal mass may have significant therapeutic implications.