SUMMARY Various aspects of pulmonary hemodynamics were studied in 34 emphysema patients: (a) when the cardiac output was interpreted on the basis of each patient's age, the majority (85 per cent) showed either normal or high output, while the remainder (15 per cent) had a definitely low output. The published data in the literature was also confirmatory of the above finding when due regard was given to the age factor; (b) the cardiac output showed a positive correlation with the pulmonary arterial pressure, but no correlation with the pulmonary resistance; (c) during exercise, the rate of increase in the cardiac output in the emphysema patients was essentially the same as that in the normal subjects; (d) the respiratory oscillation of the intrathoracic pressure was directly transmitted to the pulmonary capillary. The pulmonary arterial pressure was also greatly influenced by respiration; (e) the CO 2 inhalation (5-8 per cent) caused a significant increase in the total pulmonary resistance.