"Milk of calcium renal stone" (liquid renal calculosis) is a quite uncommon lithiasis distinguished by the presence of a semiliquid suspension of calcium salts or a "seed-like" sediment in a caliceal diverticulum or an ectasia segment of the collecting system. We reviewed 5 patients (1 male and 4 females, mean age 48.6 years), with a history of urinary tract infection, renal pain or haematuria. All patients underwent renal ultrasonographic assessment in both clinostatic and orthostatic position. Three patients underwent intravenous pyelography before ultrasound. Ultrasonography showed a sonolucent "levelled" image with a posterior acoustic shadow inside a hydro-caliceal dilation (2 pts.) or caliceal diverticulum (3 pts.); the persistence of the "level" in both clinostatic and orthostatic position allowed an immediate diagnosis in all patients. Intravenous pyelography performed before renal ultrasound showed no abnormality in 1 patient and was misleading in two; it otherwise confirmed the diagnosis when performed after renal ultrasonography. Three patients underwent surgery, two patients refused therapy; sonographic follow-up showed no evolution of the morphologic picture. Once considered as exceptional, liquid renal calculosis still remains rare pathology and accounts for 0.6% of all the urinary lithiasis diagnosed by ultrasound in our series. An accurate sonographic assessment allows a reliable diagnosis of this particular lithiasis and an easy discrimination from solid lithiasis, nephrocalcinosis, medullary sponge kidney and hydropyonephrosis. Hence, a correct diagnosis of this rare condition lets uneffective and improper treatments be avoided.