1,25-Dihydroxycholecalciferol (calcitriol) is recognized widely for its effects on bone and mineral metabolism. Epidemiological data suggest that low Vitamin D levels may play a role in the genesis of prostate cancer and perhaps other tumors. Calcitriol is a potent anti-proliferative agent in a wide variety of malignant cell types. In prostate, breast, colorectal, head/neck and lung cancer as well as lymphoma, leukemia and myeloma model systems calcitriol has significant anti-tumor activity in vitro and in vivo. Calcitriol effects are associated with an increase in G0/G1 arrest, induction of apoptosis and differentiation, modulation of expression of growth factor receptors. Glucocorticoids potentiate the anti-tumor effect of calcitriol and decrease calcitriol-induced hypercalcemia. Calcitriol potentiates the antitumor effects of many cytotoxic agents and inhibits motility and invasiveness of tumor cells and formation of new blood vessels. Phase I and II trials of calcitriol either alone or in combination with carboplatin, taxanes or dexamethasone have been initiated in patients with androgen dependent and independent prostate cancer and advanced cancer. Data indicate that high-dose calcitriol is feasible on an intermittent schedule, no dose-limiting toxicity has been encountered and optimal dose and schedule are being delineated. Clinical responses have been seen with the combination of high dose calcitriol+dexamethasone in androgen independent prostate cancer (AIPC) and apparent potentiation of the antitumor effects of docetaxel have been seen in AIPC. These results demonstrate that high intermittent doses of calcitriol can be administered to patients without toxicity, that the MTD is yet to be determined and that calcitriol has potential as an anti-cancer agent.