Using a monoclonal antibody against collagen type IV, an immunohistochemical and morphometric study was performed on trephine biopsies of the bone marrow to determine the vascular architecture of the myeloid stroma as well as the occurrence of subendothelial collagen type IV deposits. In comparison with a control group (20 patients), we investigated specimens from 20 patients with polycythaemia vera (P. vera) and 40 patients showing primary (idiopathic) osteomyelofibrosis/-sclerosis (OMF). Morphometric analysis disclosed a significant increase in the number of sinusoids and also in the extent of collagen type IV deposits in the latter disorder. Sinusoids of P. vera showed only a dilatation of their lumina corresponding with enlarged areas of sectioned vascular profiles. Contrary to this finding, in OMF sinusoidal distension was accompanied by an excessive deviation from a circular profile and prominent intravascular haematopoiesis. Furthermore, there was a significant formation of basement membrane collagen (collagen type IV). All these features were far less pronounced in early hypercellular stages of OMF than in the late fibro-osteosclerotic subtypes. Our morphometric findings confirm haemodynamic studies on advanced stages of OMF which revealed an increased vascularity. Moreover, these results are in keeping with the assumption that angiogenesis in the evolution of the fibroosteosclerotic marrow process is mediated by certain growth factors. The increase in collagen type IV formation is thought to be generated by the pronounced endothelial cell proliferation in the course of neovascularization.