Urothelial carcinomas are the most common neoplasms in the renal pelvis. However mucinous cystadenocarcinomas (MCA) are very rare in this localization. Although some theories are attributed on the patogenesis of MCA, its exact etiology is not known. Herein, we present histopathological characteristic of a case with MCA. Multiple cystic lesions and millimetric calculi with ectasia of the left kidney were detected by abdominal ultrasound and magnetic resonance imaging. Left simple nephrectomy was performed because of a pre-diagnosis of atrophic pyonephrotic kidney. The sections of the nephrectomized kidney revealed, multilocular mucinous cysts and histopathological appearance of MCA. We aimed to present this rare case mimicking atrophic cystic kidney with clinical, radiological findings, and histopathological characteristics in the lights of literature.