The widespread use of prenatal ultrasound results in an increased recognition of fetal hydronephrosis. To determine clinical characteristics and postnatal outcome of fetal hydronephrosis, we performed a retrospective study in children diagnosed as having fetal hydronephrosis between 1990 and 2001. 341 children with 427 dilated kidneys were included. Dilatation of the renal pelvis was caused by primary ureteropelvic junction obstruction in 65.6%, multicystic kidney in 9.4%, vesicoureteral reflux in 7.0%, duplex system in 5.4%, ureterovesical junction obstruction in 4.0%, and posterior urethral valves in 3.0%. Hydronephrosis resolved spontaneously in 126 (29.5%) kidneys, with 52.7% of mild hydronephrosis, and 2.6% of severe hydronephrosis. Mean interval to spontaneous resolution was 1.39 (+/- 1.41, SD) yr. Surgery was performed in 174 kidneys, including pyeloplasty in 105, ureteroneocystostomy in 23, transurethral incision in 11 and nephrectomy in 9. Most patients had initially high-grade hydronephrosis (p<0.05). Mild hydronephrosis appears to be relatively benign, and in most cases, dilatation improves with time, and thus surgical intervention is not required. On the other hand, moderate or severe hydronephrosis often results in a significantly poor outcome and requires surgical intervention, and therefore, requires closer follow-up both antenatally and postnatally.