The hemorheologic properties are obviously abnormal in patients with cor pulmonale (CP), presenting as high hematocrit (HCT), hyperviscosity and hyper-viscoelasticity. Twenty patients suffering from CP with polycythemia were randomized into either an isovolemic hemodilution (IHD) group or control group. In the IHD group, 10 patients were treated by routine therapy combined with IHD. The other 10 patients in the control group were treated by routine therapy only. The results demonstrated that after treatment in IHD group, all viscoelasticity property parameters except eta p were decreased significantly. Oxygen delivery, oxygen transport capacity, and stroke volume and cardiac output were increased. Mean pulmonary artery pressure and total pulmonary resistance were decreased. However, in the control group only HCT and eta 0.512 were decreased after treatment. The viscosity and eta' of the whole blood in IHD group were decreased more markedly than those in the control group. IHD was well tolerated in most of the patients. There were no complications or side effects. Thus, IHD is a safe, effective and acceptable therapy, and it may play a role in the treatment of cor pulmonale.