Myelofibrosis is sometimes associated with accelerated and blastic phase of chronic myelogenous leukaemia (CML). In order to investigate the role of platelet derived growth factor (PDGF) in this pathogenesis, expression and production of PDGF was studied in the blast cells from 11 patients. Five patients had myelofibrosis with myeloid blasts, while six patients did not show fibrosis, including three with myeloid blasts and three with lymphoid blasts. PDGF-A chain transcript was expressed in most of the patients. On the other hand, PDGF-B chain transcript was detected in all of the five patients with myeloid blasts and with fibrosis, in one of the three patients with myeloid blasts and without fibrosis, and in none of the three lymphoid crisis patients without fibrosis. In the patients with myeloid blasts and with fibrosis, PDGF protein, PDGF-AB and/or PDGF-BB, was found to be secreted from blast cells. In addition, the PDGF activity in the culture of myeloid blasts from two patients with fibrosis was also growth stimulatory for human marrow fibroblasts. These results suggest that expression and secretion of PDGF-AB or PDGF-BB in blast cells play an important role in the pathogenesis of marrow fibrosis associated with accelerated and blastic phase of CML.