Bone marrow angiogenesis in patients with idiopathic myelofibrosis (IM) was evaluated by using a quantitative morphometric analysis test system. IM specimens had a significantly higher degree of bone marrow microvessel density (MVD) compared with control specimens. The higher degree of angiogenesis was associated with increased expression of basic fibroblast growth factor in megakaryocytes and endothelial cells in the IM bone marrow specimens. Increased vascular endothelial growth factor expression was seen in some cases of the prefibrotic form of IM. Morphometric quantitative measurements revealed that the MVD was reduced progressively in 4 cases of IM after allogeneic stem cell transplantation. These findings suggest that angiogenesis likely has an important role in the pathogenesis of IM.