Idiopathic myelofibrosis (IMF) is a clonal stem cell disorder characterized by reactive fibrosis of bone marrow sustained by a complex cytokine network. At present, no efficacious therapy for this disease exists. Synthetic carboxy-terminal pentapeptide of osteogenic growth factor (sOGP10-14) can increase bone marrow cellularity and the number of haematopoietic colonies; this study evaluated the activity of sOGP10-14 in IMF. Fragments of bone marrow biopsies from patients affected by IMF were cultured with or without the addition of sOGP10-14. Cellular density was evaluated by image analysis, and transforming growth factor-beta1 (TGF-beta1) concentration was immunologically assayed in the supernatant of cultured bone marrow biopsies. The proliferation rate of the megakaryoblastic M07-e cell line, cultured in the presence of either granulocyte-macrophage colony stimulating factor or thrombopoietin (TPO), and with or without sOGP10-14, was evaluated. Megakaryocyte colony forming unit (CFU-Mk) assay was performed on bone marrow samples of IMF patients with or without sOGP10-14. After 14 d, bone marrow cellularity was significantly increased in samples cultured with the pentapeptide. Moreover, sOGP10-14 induced a significant increase of TGF-beta in culture supernatants. TPO-primed proliferation of M07-e was reduced by sOGP10-14, and the pentapeptide significantly reduced CFU-Mk on IMF bone-marrow-derived cells. sOGP10-14 increased ex vivo bone marrow cellularity in IMF. This action could be related to the megakaryocyte inhibition induced by the interference of this pentapeptide with growth factor activities. These findings suggest that a deficiency of osteoblast-related factors may play a role in bone marrow failure in IMF.