We report 4 cases of acute focal bacterial nephritis, a solid inflammatory lesion of the kidney. In each patient the findings on excretory urography, when combined with the clinical features, suggested the possibility of a renal abscess. Ultrasonography and computerized tomography aid in distinguishing this solid inflammatory process from that of frank abscess. The possible etiology of acute focal bacterial nephritis, its evolution to abscess formation and its distinguishing features are discussed.