Focal xanthogranulomatous pyelonephritis is an unusual form of chronic renal infection that is difficult to diagnose prior to surgery. We report on a 19-year-old woman who presented with a renal mass that mimicked malignancy. The diagnosis of focal xanthogranulomatous pyelonephritis was first suspected by radiological findings and further confirmed by histopathologic examination of percutaneous biopsy specimens of the lesion. Successful treatment of the patient was achieved with antibiotic therapy alone. Maximal efforts, including percutaneous renal biopsy, should be made to establish the diagnosis of focal xanthogranulomatous pyelonephritis before a therapeutic decision is reached. We recommend the use of antibiotics as a first-line treatment for patients with focal xanthogranulomatous pyelonephritis.