The incidence of pulmonary diseases among diabetics in Japan has been found to be more than 50% at death. This suggests the existence of some pulmonary risk factors. We analyzed pulmonary functions in 50 diabetics (31 males and 19 females) without overt lung disease, compared to 21 healthy male subjects of the same age (around 50 years old). Forced vital capacity and timed vital capacity were lower in diabetics (P less than 0.005). Diffusing capacity was also decreased in male diabetics (P less than 0.05). Among diabetics, a decrease in the diffusing capacity was dominant in patients with diabetic retinopathy, which correlated with an increasing duration of their diabetes. Analysis by the partial correlation method which accounts for the smoking index showed that standardized indices for peripheral airflow in male diabetics decreased significantly as the patient's age increased (P less than 0.005); the rate of decrement was greater than that of the control subjects. Diabetic patients showed abnormal lung function in the peripheral airways which increased with age and gas transfer was also affected by diabetic microangiopathy as well as the duration of diabetes. These changes seemed to deteriorate progressively, possibly combining and contributing to respiratory insufficiency in critical pathological conditions.