We describe a case of a monolateral duplex system and a ureterocele containing a gigantic stone in a 65-year-old woman who presented with pyelonephritis without any previous history of urinary tract infections or stone disease. Stone removal and double left ureteroneocystostomy with plastic widening of a narrowed obstructive side were performed. The ureteral stone measured 10.5 cm in greatest diameter, weighed 85 g and contained calcium oxalates and phosphates. Three months after surgery, radiology (intravenous urography and cystography) showed left unobstructed upper and lower urinary tracts and the absence of vesicoureteral reflux. Urine culture was negative 3, 6 and 9 months after surgery.