Psoriasis is associated with abnormally exaggerated epidermal cellular turnover. Recent studies showed that calcitriol (1,25-dihydroxyvitamin D3) a calcitrophic hormone, regulates terminal differentiation of basal cells of epidermal keratinocytes. We administered active forms of vitamin D3 in both oral and topical ways in an open-design study to patients with psoriasis vulgaris. Significant improvement was observed at the end of the study periods in these patients, especially in those treated with topical application of calcitriol. We also found a significant negative correlation between the severity of psoriasis and the basal serum level of 1 alpha,25-dihydroxyvitamin D but not with those of other calcium-related parameters in psoriatic patients. These data suggest that exogenous active forms of vitamin D3 are effective for treatment of psoriasis and that the endogenous 1,25-dihydroxyvitamin D level also may be involved in the development of this skin disease.