Previous in vitro and animal studies have suggested that 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] has important immunoregulatory properties. In this study, immune parameters were monitored in 16 patients the day before and 14 days after starting therapy with this hormone. Despite the absence of changes in the total white cell count, lymphocyte count and T lymphocyte count, CD4/CD8 ('helper/suppressor') ratio of T lymphocytes increased by 57% from the normal into the elevated range. Furthermore, a downward trend was seen in the percentage of stimulated lymphocytes expressing the interleukin-2 receptor. We conclude that in vivo administration of 1,25(OH)2D3 may have immunoregulatory effects in man. Whether these changes are the result of changes in intracellular or extracellular calcium concentration, parathyroid hormone concentration, or the concentration of 1,25(OH)2D3 itself remains to be answered.