Clinical and radiological observation was made on the 42 cases of childhood hydronephrosis patients admitted to Hanyang University Hospital from June, 1972 to August, 1982. Performed diagnostic methods were exccretory urography, retrograde pyelography, voiding cystourethrography and other advanced form of radiological methods such as angiography or ultrasonography. The results were as follows : 1. The male patient were more frequent than female patient and its ratio was 29 : 13. 2. 18 out of 42 patients were below 5 years of age and 9 of them were less than 1 year old. 3. Abdominal mass was the common presenting feature especially on obstructive hydronephrosis. Genitourinary symptoms or flank pain was the most frequent complaint in older children. Nongenitourinary symptoms were also noted especially in nonobstructive hydronephrosis of young children. 4. Obstructive hydronephrosis were 26 cases and nonobstructive hydronephrosis were 17 cases. Pelviureteric junction was most frequent obstruction site and intrinsic stricture was more frequent than extrinsic lesion. Vesicoureteric reflux and recurrent urinary tract infection was the frequent causes of nonobstructive hydronephrosis. 5. Both kidneys are equally affected in all of cases. The left side was more frequently affected in obstructive hydrenephrosis cases 6. Voiding cystourethrography was performed in 14 cases and showed V-U reflux in 8 cases especially of mild nonobstructive hydronephrosis. 7. Ultrasonography was performed in 10 cases and it was the good method for diagnosis especially when the abdominal mass was the presenting feature in children. 8. Most of operated cause were severe, grade III or IV, obstructive hydronephrosis cases. 9. Associated abnormalities affecting the genitourinary tract was found in 17 cases. Nongenitourinary thract abnormalities were present in 20 cases especially associated with spinal anomaly.