Simple renal cysts are acquired kidney lesion that are described as spherical, smoothed surfaced, with a serous, sub-yellow content. They are generally considered as a harmless anomaly, however cases of complicated renal cysts have been reported. We present a case of an enormous renal cyst (the biggest ever described) containing more than 25 L of fluid mimicking ascites, complicated with controlateral displacement of ipsilateral functional kidney and intra-abdominal organs, renal cell carcinoma and hypertension. Particular attention is carried in the analysis of the literature about different aspects of giant renal cysts like the ability to grow to a very giant size, the association with hypertension and renal cell carcinoma, the sensibility of the most important examinations to reveals malignancy and management.