Acute lobar nephronia (ALN) refers to a renal mass caused by acute focal infection without liquefaction. The radiological findings in 12 patients with 13 episodes of ALN are described. A characteristic combination of uroradiological findings is (a) a relatively sonolucent mass which disrupts corticomedullary definition on ultrasonography; (b) a solid-appearing mass on other uroradiological studies; and (c) a positive gallium image in the region of the mass, which may be associated with increased activity elsewhere in the same or opposite kidney. The angiographic finding of significant venous narrowing within the mass associated with only minor arteriographic abnormalities is characteristic of ALN as well.