We describe the role of US, in the diagnosis of LMPR and in differentiating LMPR from other renal disease, such as hydronephrosis and parapelvic cysts. In 10 patients mild to moderate hydronephrosis showed at the US, bilateral in 8 cases, was not confirmed at IVP and CT scan evaluation. Instead, compression of the collecting system by multiple cysts arising from the renal sinus was revealed by CT scan in 8 cases and by IVP in 2. At the U.S. the profile of the calices appeared irregular, differing from the features of hydronephrosis; furthermore calices were adjacent each other, separated only by a thin membrane. All patients were asymptomatic. The examination of the cystic liquor and wall, obtained percutaneously or during surgical procedures, showed the lymphatic origin of them. We cannot provide definitive data regarding how to differentiate LMPR from hydronephrosis at U.S.. In asymptomatic patients the U.S. evidence of dilated calices with irregular profile and thin membrane separating each other, can strongly suggest the diagnosis of LMPR.