We present a case of mucinous cystadenocarcinoma of the renal pelvis associated with a collecting duct carcinoma in a 58-year-old woman with diabetes. Even though several theories about the aetiology of mucinous cystadenocarcinoma of the renal pelvis have been proposed, its origin remains unknown. The present case shows a distinct morphology and immunohistochemical profile that may suggest a clue to its histogenesis.