Introduction Malignant cystic renal tumor is a rare variant of renal malignancy. Cystic neoplasm results from haemorrhage, necrosis and colliquation of a solid tumour or tumour occurring within the wall of a cyst. That pathoanatomic substratum reflects characteristic sonographic features indicating its malignant nature. It is important to distinguish a simple cyst (not requiring surgery) from intracystic malignant lesion because it requires surgery. Case Outline The authors present a 59-year-old woman with a sonographic finding of a simple cyst in the upper pole of the right kidney revealed during gynaecological ultrasonography. Immediately afterwards, the radiologist performed renal sonography and its finding was a cystic lesion suggestive of malignancy. Further evaluation by CT scan showed that the lesion was clearly malignant. After surgery, the histological finding verified cystic renal cancer. Conclusion Ultrasonography may reveal a complex cyst and solid mass but requires an experienced sonographer. Contrast CT scan would be performed to examine the "suspicious" lesion because it clearly shows if a cystic lesion is benign or malignant. .