The development of renal cell carcinoma in Adult Polycystic Kidney Disease (APKD) has been reported in the literature; one of the features of renal malignancy in APKD is the difficulty to make a diagnosis, and the majority of cases reported are incidental findings at surgery or autopsy. We report a rare case of renal sarcoma in a patient with APKD. Sarcoma associated with APKD does not seem to have particular biological characteristics when compared with primary renal sarcoma; however the polycystic kidney represents an aggravating circumstance, because of the difficulty in making an early diagnosis of a disease with a poor prognosis.