We evaluated the significance of lactate dehydrogenase (LDH) isoenzymes in chronic myeloproliferative disorders (CMDs) by studying LDH isoenzymes in the serum of patients with secondary polycythemia (SP), polycythemia vera (PV), essential thrombocythemia (ET) and idiopathic myelofibrosis (IMF) in different disease status. LDH activity and isoenzymes were evaluated retrospectively in serum samples from four groups of patients and compared with a control group. LDH activity and isoenzyme distributions of patients with SP and PV did not reveal significant variations with respect to controls. In the ET and IMF group LDH isoenzyme revealed significant variations: IMF showed significant increase of LDH2 and significant reduction of LDH5 isoenzyme, whereas ET showed significant decrease in LDH1 and increase of LDH3. These data suggest that LDH isoenzyme patterns may be a useful marker of CMDs, but this enzymatic pattern could be expression of a metabolic adaptation.