Platelet-derived growth factor (PDGF) has been suggested to play some role in the pathogenesis of myelofibrosis frequently encountered in patients with myeloproliferative disorders (MPD). In this study we measured PDGF activity and PF4 content in circulating platelets of patients with MPD. Both factors were lower than those of normal controls. PDGF activity in patients with myelofibrosis was slightly lower than in those without fibrosis. However, when adjusted to whole blood volume, there was a positive correlation between platelet count and PDGF activity per ml whole blood. Nevertheless, no correlation was found between activity and grade of bone marrow fibrosis. These results may support the idea that an abnormal release of PDGF occurs from platelets or megakaryocytes in the bone marrow environment, resulting in the stimulation of fibroblast proliferation, and hence, the occurrence of myelofibrosis in patients with MPD.