There is increasing evidence linking the incidence of certain cancers to low serum Vitamin D levels. The active metabolite of Vitamin D, calcitriol (1, 25-Dihydroxyvitamin D3, 1,25(OH)2D3) apart from a crucial role in maintaining mineral homeostasis and skeletal functions, has antiproliferative, apoptosis and differentiation inducing as well as immunomodulatory effects in cancer. In studying the role of 1,25(OH)2D3 in cancer, it is imperative to examine the potential pathways that control local tissue levels of 1,25(OH)2D3. The enzyme CYP24A1 or 24-hydroxylase converts 1,25(OH)2D3 to inactive calcitroic acid. Extra-renal production of this enzyme is observed and has been increasingly recognized as present in cancer cells. This enzyme is rate limiting for the amount of local 1,25(OH)2D3 in cancer tissues and elevated expression is associated with an adverse prognosis. The gene that encodes CYP24A1 has been reported as an oncogene and may contribute to tumor aggressiveness by abrogating local anti-cancer effects of 1,25(OH)2D3. It is imperative to study the regulation of CYP24A1 in cancer and especially the local metabolism of 1,25(OH)2D3 in cancer cells. CYP24A1 may be a predictive marker of 1,25(OH)2D3 efficacy in patients with cancer as an adjunctive therapy. The following review summarizes the available literature on CYP24A1 as it relates to 1,25(OH)2D3 in cancer and outlines potential ways to inhibit CYP24A1 in an effort to improve the efficacy of exogenous 1,25(OH)2D3.