Blood viscosity, pressure of pulmonary artery, pulmonary functions, arterial and mixed venous oxygen content, cardiac output, oxygen transport capacity, oxygen consumption, ejection fraction have been examined at 25 patients with or without hypoxic cor pulmonale with or without secondary polycythaemia. Although secondary polycythaemia occurs in patients with hypoxic cor pulmonale as a compensatory process to increase the oxygen carrying capacity of the blood theoretically but as our findings: 1. The rise of haematocrit causes rise in blood viscosity significantly. 2. Polycythaemia secondary over 0.50 l/l of haematocrit contributes the rise of pressure in pulmonary artery. 3. The systemic oxygen carrying capacity is increasing with the rise of haematocrit up to 0.45 l/l but decreasing over 0.50 l/l. 4. Oxygen consumption is decreasing parallel with the rise of haematocrit. 5. Pulmonary functions, cardiac output do not change by the rise of haematocrit.