Autosomal dominant polycystic kidney disease (ADPKD) is one of the most common monogenic hereditary disorders in humans characterized by fluid-filled cysts, primarily in the kidneys. Cux1, a cell cycle regulatory gene highly expressed during kidney development, is elevated in the cyst-lining cells of Pkd1 mutant mice, and in human ADPKD cells. However, forced expression of Cux1 is insufficient to induce cystic disease in transgenic mice or to induce rapid cyst formation after cilia disruption in the kidneys of adult mice. Here we report a double mutant mouse model that has a conditional deletion of the Pkd1 gene in the renal collecting ducts together with a targeted mutation in the Cux1 gene (Pkd1^CD;Cux1^tm2Ejn). While kidneys isolated from newborn Pkd1^CD mice exhibit cortical and medullary cysts, kidneys isolated from newborn Pkd1^CD;Cux1^{tm2Ejn−/−} mice did not show any cysts. Because Cux1^{tm2Ejn−/−} are perinatal lethal, we evaluated Pkd1^CD mice that were heterozygote for the Cux1 mutation. Similar to the newborn Pkd1^CD;Cux1^{tm2Ejn−/−} mice, newborn Pkd1^CD;Cux1^tm2Ejn+/− mice did not show any cysts. Comparison of Pkd1^CD and Pkd1^CD;Cux1^tm2Ejn+/− mice at later stages of development showed a reduction in the severity of PKD in the Pkd1^CD;Cux1^tm2Ejn+/− mice. Moreover, we observed an increase in expression of the cyclin kinase inhibitor p27, a target of Cux1 repression, in the rescued collecting ducts. Taken together, our results suggest that Cux1 expression in PKD is not directly involved in cystogenesis but promotes cell proliferation required for expansion of existing cysts, primarily by repression of p27.