Basic fibroblast growth factor (bFGF) is an important growth factor involved in clonal hematopoietic expansion, neoangiogenesis, and bone marrow fibrosis, all of which are important pathobiologic features of clonal chronic myeloproliferative disorders (CMPD) and myelodysplastic syndromes (MDS). The aim of this study was to assess circulating bFGF concentrations in patients with CMPD and MDS with respect to the presence of bone marrow fibrosis in histopathologic examination. The study group comprised 18 patients with CMPD (six female, 12 male; median age 50 years), seven patients with MDS (one female, six male; median age 66 years) and 10 healthy adults as controls (four female, six male; median age 29 years). CMPD group included six chronic myelogenous leukemia (CML), seven essential thrombocythemia (ET), three polycythemia vera (PV), two agnogenic myeloid metaplasia (AMM). All seven MDS patients were the FAB subtype of refractory anemia (RA). Bone marrow biopsy sections stained with hematoxylin and eosin (H & E) and for reticulin were examined for the presence of fibrosis. The median plasma bFGF level was 18.2 pg/ml (interquartile range, IQR: 15.2-26.7) in patients with CMPD, 18.0 pg/ml (IQR: 15.8-26.4) in patients with MDS, 13.6 pg/ml (IQR: 9.9-20.0) in the control group. The bFGF levels were significantly higher in patients with CMPD in comparison with the healthy control group (P = 0.031). Circulating bFGF tended to be significantly lower in relation to the development of marrow fibrosis (P = 0.028). The complicated interactions of bFGF and fibrosis in the context of CMPD may be either 'cause' or 'effect'. The bFGF might represent an important link between angiogenesis, fibrosis, and clonal neoplastic hematopoiesis during the development of CMPD.