Xanthogranulomatous pyelonephritis is a rare condition in children. A 7-year-old boy developed right flank pain and tenderness 1 month after an appendectomy. Abdominal computed tomography scan documented a right renal mass. A right nephrectomy was performed. The pathological report documents xanthogranulomatous pyelonephritis. He experienced an uneventful recovery. Xanthogranulomatous pyelonephritis should be included in the differential diagnosis of children with fever, weight loss, flank tenderness, and a renal mass.