Serum concentration of aminoterminal peptide of type III procollagen (PIIINP) were determined in 48 patients with myeloproliferative disorders (MPD). 27 with and 21 without marrow fibrosis. Serum concentration of PIIINP in MPD was significantly higher than that in control group, but not in secondary thrombocytosis. The highest serum level of PIIINP was found in the primary osteomielofibrosis. In myeloproliferative disorder a positive correlation between megakaryocyte count, marrow fibrosis and serum level of PIIINP was found. This indicates, that in MPD megakaryocytes stimulates fibroblasts to augment their fibrile formation function. Concentration of PIIINP in the serum seems to be a good, though not specific, marker of marrow fibrosis in myeloproliferative disorders.