The active metabolite of vitamin D known as 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] is a major physiologic regulator of mineral metabolism in man. The compound is also a potent inducer of differentiation of a human promyelocytic leukemia cell line known as HL-60. The induction of differentiation of myeloid leukemia cells to functional end cells offers an appealing therapeutic prospect. We investigated the ability of 1,25(OH)2D3 both to induce in vitro the differentiation of blast cells taken from patients with acute myelogenous leukemia and to improve hematopoiesis in vivo in patients with the myelodysplastic syndromes (preleukemia). We found that high concentrations (10-6 M) of 1,25(OH)2D3 significantly induced the in vitro differentiation of blast cells as measured by morphology, phagocytosis, and superoxide production. A concentration of 10-9 M 1,25(OH)2D3 had no effect on blast cell differentiation. We gave 2 microgram/day of 1,25(OH)2D3 to 18 patients with myelodysplastic syndrome (preleukemia) in an attempt to improve their hematopoiesis. During therapy, their peak peripheral blood granulocyte, platelet, and macrophage concentrations were slightly elevated as compared to their baseline, starting levels. Eight patients had a partial or minor peripheral blood response to the compound during the administration of 1,25(OH)2D3. However, no patient showed significant improvement of peripheral blood cell or marrow blast cell counts by the end of the study (greater than or equal to 12 weeks) as compared to their starting levels. Seven of the patients developed leukemia before or by 12 weeks of treatment. Nine of the 18 patients developed hypercalcemia. Taken together, the study shows that high concentrations (10-6M) of 1,25(OH)2D3 can induce differentiation of leukemia blast cells in vitro, but the administration of 1,25(OH)2D3 to patients with the myelodysplastic syndromes (preleukemia) does not have an enduring therapeutic effect. Hypercalcemia prevented administering greater amounts of 1,25(OH)2D3. In the future, the use of new vitamin D analogs that induce hematopoietic cell differentiation without inducing hypercalcemia might allow the achievement of higher blood levels of the inducing compound and might be medically useful for selected preleukemic and leukemic patients.