The INK4a proteins p16^INK4a and p19^ARF regulate cell cycle arrest and senescence. However, the role of these proteins in controlling these processes in the normal kidney and following injury is unknown. We performed unilateral ureteral obstruction (UUO) to induce fibrosis in 2- to 3-mo-old wild-type (WT) C57/B6 and INK4a knockout mice. By quantitative RT-PCR, p16^INK4a levels were increased sixfold in WT mice 7 days after UUO and p19^ARF remained undetectable. Kidney sections were examined to determine levels and localization of p16^INK4a, apoptosis, fibrosis, and senescent cells. INK4a knockout mice displayed mesangial cell proliferation, increased matrix deposition, and myofibroblast differentiation under normal conditions. Following UUO, INK4a knockout mice displayed 10-fold increased tubular and interstitial cell proliferation, 75% decreased collecting duct apoptosis, 2-fold greater collagen and fibronectin deposition, and no cell senescence by senescence-associated β-galactosidase staining compared with WT mice. Both INK4a knockout mesangial cells and kidney lysates from knockout mice following injury showed elevated levels of IL-6 by ELISA compared with WT samples. INK4a knockout epithelial cell cultures displayed increased mesenchymal cell markers when exposed to transforming growth factor-β. These results confirm that p16^INK4a controls cell proliferation and matrix production and mitigates fibrosis following injury and suggest that the mechanism involves a role in limiting inflammation and cell proliferation.