Whereas beta-2-microglobulin (beta2M) has mainly been used as a prognostic factor in patients with lymphoproliferative disorders, some studies have reported the value of beta2M in myeloproliferative disorders (MPD). In order to investigate a potential role in the pathogenesis of MPD and to find a possible value as indicators in monitoring the course of the disease, we measured beta2M, TNF-alpha, IL-1alpha, IL-1beta, IL-2, sIL-2R, IL-6 and IL-10 in 55 patients with MPD, at diagnosis and during the course of the disease. In progressive disease and particularly when transformation to acute leukemia occurred, high levels of beta2M, IL-2 and sIL-2R were found in all patients; the elevation was progressive, which suggests a potential prognostic usefulness in the individual patient.