Autosomal dominant polycystic kidney disease, the most common monogenetic disorder, is characterized by gradual replacement of normal renal parenchyma by fluid-filled cysts. Mutations in either PKD1 or PKD2 cause autosomal dominant polycystic kidney disease. Pkd1(-/-) or Pkd2(-/-) mice develop rapid renal cystic disease and exhibit embryonic lethality; this supports the "two-hit" hypothesis, which proposes that a germline mutation in PKD1 (or PKD2) followed by a second somatic mutation later in life is responsible for the phenotype. Here, for investigation of the loss of Pkd1 at specific times of development, an inducible Pkd1-knockout mouse model was generated. Inactivation of Pkd1 in 5-wk-old mice resulted in formation of only focal renal cysts 6 to 9 wk later but in a severe polycystic phenotype nearly 1 yr later. Cysts derived from either collecting tubules or distal tubules but not from proximal tubules, which correlated with sites of Cre-mediated recombination. Inactivation of Pkd1 in 1-wk-old mice, however, resulted in massive cyst disease 6 wk later, despite a similar pattern of Cre-mediated recombination between 1- and 5-wk-old kidneys. Moreover, a germline heterozygous Pkd1 mutation facilitated cyst formation when a somatic Pkd1 mutation was induced. A marked increase in proliferating cell nuclear antigen expression was observed in cyst-lining epithelia and in normal-looking tubules adjacent to but not in those distant from cysts. These data suggest that Pkd1 inactivation is not sufficient to initiate the cell proliferation necessary for cyst formation; a paracrine mechanism may account for focal cell proliferation and regional disease progression. We propose that an additional genetic or nongenetic "third hit" may be required for rapid development of cysts in polycystic kidney disease.