Congenital hydronephrosis is relatively common abdominal mass in infants and children. But report of congenital hydronephrosis in twin was not found. We have experienced case of congenital giant hydronephrosis in a male twin pair. Twin A at 3 years of age and twin B at 4 years of age, who had huge abdominal mass on the left side. So we have reported them with the clinical, radiological and pathological findings and reviewed references.