Xanthogranulomatous pyelonephritis is a form of chronic infection of the kidney and surrounding tissues characterized by destruction and replacement of renal parenchyma by lipid-laden macrophages. Gross pathologic features include massive renal enlargement, lithiasis, peripelvic fibrosis, hydronephrosis, and lobulated yellow masses replacing renal parenchyma. Typically, the disease is diffuse and has characteristic imaging features. Less commonly, the process is focal and is difficult to differentiate from malignant disease on radiologic studies. Ultrasound demonstrates renal enlargement with multiple anechoic or hypoechoic masses replacing the normal corticomedullary differentiation and a contracted pelvis. Peripelvic fibrosis may obscure acoustic shadowing from a central staghorn calculus. On computed tomographic scans, a staghorn calculus may be seen in a contracted renal pelvis of an enlarged kidney, with characteristic low-attenuation, peripherally enhancing rounded masses. Extrarenal extension of the inflammatory process is frequently seen.