Multicystic Renal Dysplasia (MRD) was discovered during antenatal ultrasound examination in 138 fetuses between 1980 and 1995. Associated malformations were present in 66 % (42 % urological) and 22 % of the fetuses did not survive the pregnancy or the peri-natal period.Anatomical analysis showed a wider variety of MRD than in classical descriptions. Obstruction of the urinary tract was almost invariable. Like the hypothesis published by Beck in 1971, our view is that, with a very early obstruction of the urinary tract (during the first trimester), there is a dysplastic evolution of renal tissue, while later in pregnancy the same obstruction can induce a hydronephrosis with corticomedullary dysplasia.We advise complete neonatal urological investigation, and surgical removal of multicystic kidneys, to avoid multiple and inadequate evaluations of those children with a single functioning renal unit.