It has been proposed that Vitamin D has a significant influence on disease progression in malignancy. This study aims to investigate whether serum levels of 25-hydroxyvitamin D 25(OH)D are associated with prognosis in patients with hematological malignancies. This study is based on 105 patients with hematological disease (acute and chronic leukemias, myelodysplastic syndromes, monoclonal gammapathies, and chronic lymphoid disorders), seen over a 6-months period. 25(OH)D deficiency (<20 ng/ml) appeared very common and an inverse relationship was observed between 25(OH)D levels and the response to therapy: lower levels being related to poorer response. In acute leukemias, a significant difference was noted between patients with long-term disease-free survival in those tested at diagnosis (P = 0·001) or in those tested at the time of relapse (P = 0·05). Similarly in patients with Philadelphia-positive leukemias, there was a correlation between molecular response and levels of 25(OH)D (P = 0·01). Previously identified factors, such as age, season, gender, or nutritional index, were not related to circulating 25(OH)D levels. Lower levels of circulating 25(OH)D appeared related to a progressive stage of the disease and poor response to therapy, and, therefore, to the aggressiveness of the disease. It is a potential marker of prognosis in patients with leukemia.