(a–d) I.V. contrast-enhanced CT abdomen and pelvis demonstrating extensive bilateral gaseous replacement of the normal renal parenchyma and perinephric space, consistent with emphysematous pyelonephritis. Free gas can be seen to track within the retroperitoneum, extending superiorly into the mediastinum and inferiorly along the left paracolic gutter and into the pelvis. (a) and (c) are set on soft tissue windows. (b) and (d) are on lung windows.