The serum concentration of hyaluronan (HYA) was determined in 59 patients with various myeloproliferative disorders, including 33 patients with idiopathic myelofibrosis. In 18 patients the serum concentration of the aminoterminal propeptide of type III procollagen (PIIINP) was measured concomitantly. Raised serum HYA levels were seen in patients with active disease compared with age‐matched healthy subjects, whereas no significant difference in serum HYA was seen between patients with stable disease and age‐matched controls. Serum HYA concentrations correlated significantly with the leukocyte count (rho = 0.38; P < 0.02) and with the serum concentration of PIIINP (rho = 0.50; P < 0.001). During cytotoxic treatment, the serum HYA and PIIINP concentrations decreased in concert with declining leukocyte counts. These findings suggest that clonal expansion is accompanied by a bone marrow stromal reaction similar to the repair processes following injury to soft connective tissues. The relatively modest changes in serum HYA with frequent overlaps between patient categories and healthy subjects imply that the clinical utility of single determinations of serum HYA in the present disease groups is restrained. On the other hand, sequential measurements of HYA may provide a reflection of the myeloproliferative process in individual patients.