MED1 (mediator complex subunit 1) is expressed by human epidermal keratinocytes and functions as a coactivator of several transcription factors. To elucidate the role of MED1 in keratinocytes, we established keratinocyte-specific Med1-null (Med1epi-/-) mice using the K5Cre/LoxP system. Development of the epidermis and appendages of Med1 epi-/- mice were macroscopically and microscopically normal until the second catagen of the hair cycle. However, the hair cycle of Med1 epi-/- mice was spontaneously repeated after the second telogen, which does not occur in wild-type (WT) mice. Hair follicles of Med1 epi-/- mice could not enter anagen after 6 months of age, resulting in sparse pelage hair in older Med1epi-/- mice. Interfollicular epidermis (IFE) of Med1epi-/- mice was acanthotic and more proliferative than that of WT mice, whereas these findings were less evident in older Med1epi-/- mice. Flow cytometric analysis revealed that the numbers of hair follicle bulge stem cells were reduced in Med1epi-/- mice from a few months after birth. These results suggest that MED1 has roles in maintaining quiescence of keratinocytes and preventing depletion of the follicular stem cells.