Giant hydronephrosis in adults is a rare entity. It is defined as an extensive dilatation of the pyelocaliceal cavities occupying a large part of the abdominal cavity. Giant hydronephrosis is usually due to pelvi-ureteric junction obstruction and is usually diagnosed in children and infants. Ureterocele, which is a cystic dilatation of the terminal ureter, often drains the upper part of the kidney in patients with a duplex system. Massive hydronephrosis in a patient with duplex system and obstructive ureterocele was described by Aeron et al. in 2017. A thorough search of the major medical databases disclosed that no other cases have been reported since. We describe a second case of unilateral complete duplex system with ureterocele and massive hydronephrosis of the upper moiety in an adult man with intermittent abdominal pain associated with constipation and a decrease in appetite. The renal function of the left kidney was 8% of total function by radionuclide renal scan. The patient subsequently underwent left laparoscopic nephrectomy.