The relationship between the extent of bone marrow reticulin and collagen fibrosis and the concentration of granulocytic (CFU-GM), erythroid (BFU-E) and megakaryocyte (CFU-Mk) progenitor cells in the peripheral blood of patients with primary agnogenic myeloid metaplasia (AMM) and secondary myelofibrosis (sMF) has not been definitively correlated. We studied 23 patients with established diagnosis of AMM and 12 patients with sMF for the extent of reticulin and collagen bone marrow fibrosis and for the spontaneous colony (sCFU-GM, sBFU-E and sCFU-Mk) formation. The control group consisted of 11 healthy volunteers. Trephine biopsy of the posterior iliac crest was performed in all individuals studied to determine the type and degree of reticulin and collagen fibrosis. Gomori's silver impregnation technique was used. sCFU-GM, sBFU-E and sCFU-Mk colony formation was related positively to spleen size, the white blood cell counts and the degree of collagen fibrosis in AMM (p < 0.01). Stimulated CFU-GM were also significantly correlated with the degree of bone marrow reticulin and collagen fibrosis. There was no correlation between the extent of peripheral blood progenitor concentration and the degree of bone marrow reticulin or collagen fibrosis in sMF and in control individuals. In conclusion, the extent of bone marrow fibrosis is significantly correlated with the peripheral blood progenitor colony formation in AMM but not in sMF.