PURPOSE: To establish the standard guideline for differentiating the extrarenal pelvis from obstructive uropathy on fetal ultrasonogram (US) to avoid unnecessary postnatal follow-up and other additional examinations. MATERIALS AND METHODS: From July 2000 to July 2001, thirty-four kidneys with hydronephrosis diagnosed on fetal ultrasonogram performed during the third trimester of pregnancy were included in this study. Hydronephrosis was defined as the pelvic anteroposterior (AP) diameter being 4 mm or greater before 33 weeks of gestation while 7 mm or greater at or after 33 weeks of gestation. The size of the renal pelvis was measured at intrarenal, intra-extrarenal junctional and extrarenal portions in every kidney on the transverse view of the fetal renal hilum. Postnatally, all neonates underwent renal ultrasonogram 2 to 8days after birth, and renal pelvic diameters were measured using the same method as the fetal US in 28 kidneys. We then compared the extrarenal-intrarenal ratio (E/I ratio) of pelvic diameter between fetal and neonatal kidneys. We presumed that the extrarenal pelvis on fetal US was the pelvis showing the normal intrarenal pelvic diamater accompanied by the most dilated extrarenal pelvic diameter. Follow-up ultrasonograms were performed in 12 of 17 neonates who had the maximal diameter at extrarenal portion on fetal ultrasonogram. VCUG and IVU were taken in 2 patients with a persistent dilatation of the renal pelvis on follow-up ultrasonograms. RESULTS: On fetal US, 17/34 kidneys showed the extrarenal portion with the most dilatation while in 12/34 kidneys, the intra-extra renal junction portion was the most dilated portion. Meanwhile, 2/34 kidneys showed the intrarenal portion with the most dilatation while other 2/34 kidneys showed dilated intrarenal and juctional portions but the same diameter. The remaining one patient showed the dilated extrarenal and juctional portions with the same diameter. The E/I ratios of fetal and neonatal pelvic diameters showed a strong positive linear correlation. Neonatal and follow-up US of extrarenal dilatation in three kidneys before 33 weeks showed normalization of renal pelvis. Among 14 kidneys with extrarenal dilatation before 33 weeks of gestation, postnatal ultrasonogram revealed the normal pelvic diameter in seven, persistent extrarenal pelvis in one, ureteropelvic obstruction in two, persistent calyectasis in two, and persistent pyelectasis in two patients. Six of seven kidneys with dilated extrarenal and intrarenal pelvis less than 5mm showed the normal diameter on the postnatal exams whereas one patient showed the persistent extrarenal pelvis. CONCLUSION: If the intrarenal portion of the fetal renal pelvic diameter is less than 5 mm and if there is a presence of combined dilatation of the extrarenal portion of the fetal renal pelvis on third trimester ultrasonogram, it is more likely either the transient extrarenal dilatation of normal renal pelvis or neontal extrarenal pelvis. When this guideline is applied, unnecessary postnatal follow-up studies may be avoided.