A 72-year-old male patient presented to the hospital with right flank pain, fever and haematuria. Computed tomography revealed a horseshoe kidney, grade 2 hydronephrosis and a focal hyperdense lesion, of which differential diagnosis between mass and abscess could not be made, in the right kidney. Magnetic resonance imaging was performed subsequently and revealed a mass. Right radical nephroureterectomy was then performed. Pathological examination described a fragile, necrotic lesion, which was a sign of chronic pyelonephritis that dilated the pelvicalyceal system. It was also associated with the mucosa and formed cystic structures with a size of 5×4 cm in the cortex. Chronic pyelonephritis cases with acute episodes should be considered in the differential diagnosis of renal tumours.