Abstract.Psoriasis is a genetically determined chronic disease of the skin, and an accelerated proliferation of epidermal keratinocytes is one of the pathophysiological characteristics of psoriatic lesion. Recently, it was reported that topical administration of 1,25‐dihydroxyvitamin D3was effective for psoriasis. Our study was done to investigate the effect of 1,25‐(OH)2‐D3on cell kinetics on human epidermal keratinocytes as a possible mechanism of its effect on psoriasis.After 24‐h application of 1 μg g–11,25‐(OH)2‐D3ointment to psoriatic lesion, the number of mitotic keratinocytes decreased. When the cultured human keratinocytes were exposed to 10–8mol l–11,25‐(OH)2‐D3for more than 9 days, inhibition of cell proliferation was noted. DNA distribution analysis by flow cytometry showed a decrease in cells in the S phase, and increase in 2c cells. This indicates blockade of the cell cycle in the G1phase. The cell cycle time was not extended as a result of 1