Fifty-one patients with acute inflammatory renal disease were examined with ultrasound. The spectrum of findings seen in acute pyelonephritis is described, with emphasis on the evolution of the disease from acute focal inflammation to perinephric extension. There were 26 cases of acute bacterial nephritis, 15 cases of abscess formation, and 10 cases of pyohydronephrosis. Pathologic correlation was obtained. The impact of percutaneous needle puncture on the diagnosis and treatment of all stages of renal disease is discussed. Prompt diagnosis can be made, and specific antibiotic therapy can be instituted on the basis of information obtained by ultrasound examination and percutaneous aspiration. If necessary, treatment may be aided by ultrasonically guided placement of drainage catheters. Serial examination will show resolution of the disease, and it avoids exposure of the patient to ionizing radiation and intravenous contrast agents.