Renal cysts are present in a wide variety of renal diseases, including those having a dysplastic nature and those of genetic origin. Genetic counseling requires clear differentiation between these types of cysts. This concept has made the classification of Potter useless because of the confusion it introduces between inherited diseases and developmental abnormalities. According to this classification, type I corresponds to autosomal recessive polycystic kidney disease (RPKD), type II to multicystic dysplasia, type III to autosomal dominant polycystic kidney disease (DPKD), and type IV to cystic dysplasia associated with urethral obstruction. The term "polycystic kidney disease" should be reserved for two hereditary cystic diseases, RPKD and DPKD, which are clearly distinct both by their modes of inheritance and by their pathologic characteristics. Both forms of polycystic disease may occur in children, but because the recessive form is much more prevalent, it is commonly called the "infantile form," whereas dominant polycystic disease is commonly known as the "adult form." The pediatric expression of these two forms and the problems of differential diagnosis will be examined.