The classical role of calcitriol (1a.,25- dihidroxivitamin D3) on calcium homeostasis is rather well known. Recent findings, of the calcitriol receptor on monocytes, activated lymphocytes and other cell Iines, and the ectopical and localized hormone production by macrophages, suggest that this hormone is playing a new and important role in the modulation of immunological responses. These data support the concept that this hormone, first considered to be an immunosuppressor, could activate, as well as suppress, the immune function, depending on the Iymphocyte activation mechanisms and the immunological microenvironment. The c1inicalinformation available and the in vivo and in vitro experimental evidence, support the concept of an immunoregulatory role for calcitriol. Hence the hormone should be considered a therapeutical immunemodulalor.