Excessive proliferation of epidermal keratinocytes is a typical aspect of chronic skin diseases such as psoriasis. In the present study, the effect of phosphodiesterase 7A (PDE7A) inhibitor ASB16165 on proliferation of keratinocytes was investigated to examine the role of PDE7A in keratinocyte proliferation and the possible therapeutic relevance of PDE7A inhibition in psoriasis. Topical application of ASB16165 inhibited the increase of thickness of skin as well as epidermis in a skin inflammation model induced by repeated painting of 12-O-tetradecanoylphorbol-13-acetate (TPA) in a concentration-dependent manner. The ASB16165 treatment also suppressed the increase in the number of Ki67-positive keratinocytes in the model, showing the disturbance of keratinocyte proliferation by the treatment. In addition, both ASB16165 and dibutyryl cAMP significantly decreased the proliferation of human keratinocytes in vitro, suggesting that PDE7A participates in keratinocyte proliferation probably by controlling intracellular cAMP, while the contribution of other mechanism(s) is not completely denied. The findings in the present study indicate that the effect of ASB16165 on skin and epidermal hyperplasia in the TPA-induced skin inflammation is mediated, at least in part, by the inhibition of keratinocyte proliferation. The inhibitors for PDE7A including ASB16165 might be useful for the treatment of psoriasis.