Clean and dirty shadowing are common phenomena in ultrasound (US) imaging. Clean shadowing is thought to be produced by sound-absorbing materials (ie, stones), and dirty shadowing is thought to be produced by sound-reflecting materials (ie, abdominal gas), but these properties are not consistent. To evaluate the characteristics of shadows behind different objects at US, the authors scanned two renal stones and a bovine femur (each of which had part of its surface artificially smoothed or roughened) and a tissue-mimicking phantom comprising air-containing cylinders of different radii of curvature. The rougher and/or smaller the radius of curvature of the surface insonified by the sound beam, the cleaner was the shadow, independent of the composition of the underlying reflecting medium. Clean and dirty shadowing were primarily related to the properties of the surface of the shadowing object and provided little information about the structural nature of the object.