The pulmonary function and microscopic change of the lungs of diabetic patients were examined and compared with those of non-diabetic patients to assess the diabetic microangiopathy in lung. For pulmonary function study, spirogram flow-volume curve, diffusing capacity and arterial blood gas analysis were performed in 52 diabetic patients and 48 age- and sex-matched control subjects. Diffusing capacity, % vital capacity, total lung capacity, residual volume and 25% maximal expiratory flow were significantly less in the diabetic group than in the control group. PaO2 was also decreased in the diabetic group. There were no significant differences between the two groups in the other parameters. For histopathological study, the lungs of 35 autopsied cases of a diabetic group and 26 autopsied cases of a non-diabetic group. There were no significant differences in age and sex between the two groups. The two groups were compared and studied by measuring the thickness of alveolar capillary walls, pulmonary arteriolar walls and alveolar walls with a light microscope and an eye piece micrometer. The alveolar capillary walls, the pulmonary arteriolar walls and the alveolar walls had thickened significantly in the diabetic patients. These studies suggested that histological changes (microangiopathy) in the lungs are a cause of pulmonary function abnormalities.