A uremic patient developed hypercalcemia after tuberculosis infection, and his ionized calcium levels correlated with 1,25-dihydroxyvitamin D3(1,25(OH)2D3) levels. We performed further studies to determine whether monocytes are alternative sites of 1,25(OH)2D3conversion beyond renal tubular cells. Using an ex vivo bioassay, in this study, we found that 1-αhydroxylase (CYP27B1) activity in monocytes is significantly higher in patients with active tuberculosis (TB) than in those with frequent TB contact. However, when monocytes from patients with active TB were restimulated with antigen derived fromMycobacterium tuberculosis, less 1,25(OH)2D3was observed. In contrast, the level of 1,25(OH)2D3was unchanged in those with frequent TB contact. We conclude that monocytes may be an alternative source of 1-αhydroxylase that could convert 25-hydroxyvitamin D3to the more active 1,25(OH)2D3.