SUMMARY 1.Measurements of bronchial blood flow were performed by the dye dilution method in 50 patients with various chronic pulmonary diseases including bronchiectasis, pulmonary tuberculosis, silicosis, pulmonary abscess, pulmonary emphysema, and pulmonary neoplasms, and the influences of bronchial circulation on respiration and circulation were studied. 2.In bronchiectasis, the bronchial blood flow showed the greatest increase among these pulmonary diseases and a surprising large value was obtained in one case. 3.In six of eight cases with pulmonary tuberculosis, no increase in bronchial blood flow was observed, but in the other two significantly increased flow was found. In silicosis, the cases with simple silicotic nodules did not show increased flow, while the cases with massive densities proved the significantly increased flow. We were unable to clarify the participation of tuberculous complication to the development of the bronchial collateral vessels. 4.Seven of eight cases with pulmonary emphysema did not indicate any increment in bronchial blood flow, but one with long-standing chronic bronchitis showed slightly increased flow. In pulmonary abscess the increase was insignificant in all four cases. 5.In pulmonary neoplasms, four of five cases with primary bronchogenic carcinoma showed increased bronchial blood flow, while two with metastatic pulmonary tumor did not show this increased flow. However, one case of metastatic lung cancer with an episode of pneumonia showed a large increase in bronchial blood flow. 6.From these results it was deduced that pulmonary infection is a contributing and determining factor in the development of the bronchial vessels. 7.When the bronchial blood flow was increased, irrespective of the cause, oxygen saturation and venous admixture ratio were maintained near the normal value. On the other hand, heart size on teleroentgenogram was enlarged in the cases with greatly increased bronchial blood flow, in the absence of systemic hypertension and without correlation with the elevation in pulmonary artery pressure.