We have wanted to appraise the diagnostic reliability of fetal ultrasonography (FU) and its correlation with postnatal ultrasonography (PU). For two years we have studied 20.506 children born in 12 Pediatric Division of Veneto, all provided with FU. Between the 23rd and the 40th week of gestation the FU has individualized 177 fetuses with pelvic dilatation. The PU, performed in the 2nd and 7th day of life and then repeated to 1 month of life, has confirmed the prenatal diagnosis in 150 of the 247 renal unities, we have observed the 39% of false positive and the 8% of false negative. The sensibility has been 94%, specificity of 46%. The definitive diagnosis was: mild pelvic ectasia in 45%, ureteropelvic junction stenosis in 26%, vescicoureteric reflux in 15%, duplication of the renal collecting system and primary megaureter in 3%, multicystic kidneys and urethral valves in 5%. 20 children have been submitted to surgical corrective intervention. From the correlation performed between the entity of pelvic dilatation to the FU and to the PU we think it advisable to submit all the fetuses that show a caliceal dilation of > 5 mm to two ultrasounds, the first one between the 2nd-5th day of life and the second one at 30 days of life. In the newborn with caliceal dilatation of > 10 mm a micturating cystogram must be performed and subsequently in case of vescico ureteric reflux a DMSA scan is advisable, while in absence of reflux a DTPA scan or MAG3 scan is necessary. The renal pelvis with a diameter between 5-10 mm will perform ultrasound controls only each three months, unless there are partial or total reductions of the renal parenchymal, dilatation of the calices and/or the ureters.