In 23 patients with anemia or polycythemia but normal lung function, a statistically significant correlation between hemoglobin concentration in the blood -and the diffusing capacity of the lung for CO was found. A change of 1 g hemoglobin per 100 ml blood changes blood the diffusing capacity for about 6.5 % of the predicted value. The diffusing capacity of the lung membrane will be less than 50 % of the total lung capacity with decreasing hemoglobin concentration in the blood. This result confirms the theory of Koyama and Mochizuki [1969] that the resistance to diffusion of the lung membrane is a function of the hemoglobin concentration of the lung capillary blood.