An extracapsular, hypoechoic or anechoic rim of simple-appearing fluid surrounding the kidneys, first described on ultrasonography in some patients with renal failure (serum creatinine >2 mg/dL), was termed kidney sweat. The rim is thought to represent perirenal edema, and the differential includes more significant perirenal fluid collections, such as hematoma or abscess. It can also be identified on CT and MR scans, with typical imaging features of simple fluid. The amount of fluid varies and when a large volume of fluid surrounds the kidney, the term ‘floating kidney’ has been applied. The pathogenesis and prognostic significance of this finding remain unclear. It can be seen both in chronic kidney disease and acute kidney injury.