The large diversity of genetic and nongenetic causes of polycystic kidney disease begs the question of whether there is a common denominator in the process of cyst formation. Accumulating evidence now indicates that cysts are made up of immature epithelium, which suggest that cystic transformation may be caused by an arrested developmental process that locks cells in a unique differentiated state. Although there may be any number of possible causes of this cystic change, it is proposed that one unifying mechanism may be subcritical injury-induced cellular dedifferentiation.