The systematic prenatal diagnosis of 5 cases of urinary tract malformations out of 2 500 pregnancies in 1982 in a special care maternity, or 2 per 1 000 is compared with the discovery of 10 cases in 28 months in 17 960 foetuses, or 0.55 per 1 000 whereas the overall frequency varies from 0.3 to 5.25 per 1 000 in infants. We studied 13 children whose prenatal abnormalities were 2 unilateral ureterohydronephrosis and 7 bilateral dilatations associated to 2 ureteroceles, 1 multicystic dysplasia and 1 ureteral duplicity. All of these except two were confirmed at birth. The exceptions consisted in one unilateral dilatation and one bilateral ureterohydronephrosis which turned out to be dysplastic kidneys unknown during pregnancy. Three ureteral duplicities were also ignored. The results are the following: Three cases were severe: one therapeutic abortion after 27 weeks, one intra-uterine decompression followed after few days by a preterm delivery and neonatal death and one provocated preterm delivery (36 weeks). All the others underwent decompression during the first hours of their life. Five cases can be considered unsuccessful: 2 abortions, the death of a 22 days old infant with an unilateral multicystic dysplasia and 2 bilateral malformations with renal failure, one of which was associated to an unilateral dysplastic kidney. Reliability of sonography as well as its interest in prognosis and prenatal evaluation of renal function are demonstrated.