OBJECTIVE: To help prenatal counselling in fetal hydronephrosis by demonstrating the postnatal investigation, treatment and outcome of infants with hydronephrosis prenatally diagnosed. METHODS: Between January 2000 and December 2001, we studied 20 infants who presented with fetal hydronephrosis confirmed by postnatal ultrasonography. In the postnatal follow-up period, the infants were followed with sequential ultrasonography and urinalysis. (99m)Tc-DTPA scan, intravenous pyelography and voiding cystourethrography were performed in selected cases. An anteroposterior renal pelvic diameter >7 mm after 30 weeks of pregnancy was defined as fetal hydronephrosis. Follow-up ranged from 6 to 18 months (mean, 12). RESULTS: Unilateral hydronephrosis was diagnosed in 13 infants and bilateral in 7. A male predilection was found (4:1) and the left kidney was more commonly involved. If there was no resolution, ultrasonographic follow-up was done until 18 months. As a results, hydronephrosis resolved in 11, who were all in the unilateral hydronephrosis group. The range of the fetal renal pelvis on prenatal ultrasonography was 7~13 mm in the resolution group. Pyeloplasty was performed in two unilateral hydronephrosis infants. CONCLUSION: When the fetal renal pelvis was below 14 mm at least on prenatal ultrasonography, it didn't progress. Fetal hydronephrosis below 14 mm may be safely observed, and surgical correction was performed only a few infants. So, we suppose that this outcome must be considered enough in prenatal counsellings and that the work-up for more many people is needed, because of the small number of the patients whose renal pelvic diameter is above 14 mm in this study.