The properties of human bone marrow fibroblastoid colonies (CFU-F) were studied in normal subjects and in patients with myeloproliferative disorders. Colony incidence was within normal values in all groups of patients analyzed except for myelodisplastic syndromes, with higher mean value. The growth rate of CFU-F is inversely related to the initial colony-forming efficiency both in normal subjects and in patients. Direct correlation between CFU-F and granulocyte-macrophage colony-forming unit (GM-CFU) was detected only in normal subjects, but lacked in patients. Higher number of CFU-F was observed in subjects with increased incidence of bone marrow megakaryocytes or peripheral blood platelets, irrespective of the underlying disorders and the platelet-derived growth-factorenhanced cloning efficiency of bone marrow fibroblasts.