Cdc42 is a small GTPase, which acts as a molecular switch to regulate a wide variety of cellular functions, such as actin cytoskeleton organization, cell proliferation, apoptosis, cell migration and in particular, cell polarity. Formation and maintenance of the basement membrane is a polarized process, which requires directed secretion, deposition and organization of basement membrane components at the basal side of epithelial cells. In the current study, we analyzed the maintenance of skin basement membrane in mice with a keratinocyte-restricted deletion of the Cdc42 gene. In the absence of Cdc42, basement membrane components became aberrantly deposited and the processing of laminin 5 was impaired in parts of the dermal-epidermal junction. These impairments became more severe with age and corresponded to local defects of the basement membrane in 4.5-month-old mutant mice. However, both, structure and number of hemidesomosomes were not significantly changed in the Cdc42 mutant skin compared with the control mice and no blister formation was observed in mutant skin. These data indicate that Cdc42 in keratinocytes is important for maintenance of the basement membrane of skin.