A case of congenital, unilateral giant hydronephrosis containing 42 1/2 litres of urine in an adult male is reported. Diagnosis and management are discussed. Because of the possible effects of sudden abdominal decompression, a plea is made for a two-stage procedure. The hydronephrosis is first decompressed slowly by drainage and nephrectomy is then performed after the patient's cardiorespiratory and alimentary systems have been stabilised.