If a slightly enlarged left kidney with separation of the collecting system is abnormal in an adult, it suggests that hydronephrosis may be present, possibly due to urinary tract tumors, stones, hyperplasia, or congenital developmental anomalies. In some fetuses, if renal ultrasonography is performed, physiologic separation of the collecting system is seen, which usually resolves on its own. On renal ultrasonography, if a separation of more than 0.9 cm occurs between the mucosa on the surface of the renal pelvis inside the kidney, it is called a separation of the collecting system, and the cause is mostly due to hydronephrosis, which also leads to an increase in the size of the kidneys. This sign is abnormal and further examination of the cause of hydronephrosis formation is recommended. The formation of an enlarged left kidney with separation of the collecting system may be due to conditions such as developmental malformation narrowing of the ureteral junction of the renal pelvis, vesicoureteral reflux, tumors of the renal pelvis or urethra, stones, and so on. However, it should be noted that in some fetuses, if renal ultrasonography is performed, physiologic separation of the collecting system can be seen, which may be accompanied by a slight enlargement of the kidneys. This abnormality is reproducible, and may resolve spontaneously as the fetus develops or after birth. Adults with these abnormalities are advised to seek prompt medical attention and treatment as prescribed by the doctor.