Examination of blood rheologic properties in 38 patients with chronic postembolic pulmonary hypertension revealed an elevated blood viscosity at all shift rates and a reduced erythrocyte deformability. A close positive correlation was found between blood viscosity and perfusion deficit. Hemorheologic disorders in patients with chronic postembolic pulmonary hypertension are caused by pachyhemia and blood acid-base imbalance due to respiratory failure. They deteriorate pulmonary and systemic hemodynamic disturbances and contribute to progression of blood flow derangement.