Background. Diabetic angiopathy has been reported to be a common complication in patients with diabetes mellitus. The lungs also have been reported to be the affected organ. The present study aimed to determine the impacts of glycemic control on lung function in younger and older patients with diabetes. Methods. One hundred twenty‐two patients were recruited to the present analytical observational study. HbA1c, patient’s medical history, and current health status obtained from the hospital medical records were collected. Spirometry was performed for the lung function test. Self‐reported respiratory symptoms were obtained. Regression analysis, odds ratio, and mean differences between groups were revealed. Results. Forced vital capacity (%predicted) in patients with diabetes was lower than 80%. It also was associated with the self‐reported respiratory symptoms. Regression analysis revealed that glycemic control influenced pulmonary function in patients with diabetes (standardized coefficient beta = −0.224, p value = 0.014). HbA1c level > 7% was a factor in reducing the forced vital capacity in the patients (odds ratio = 3.125; 95% CI, 1.401–6.964). With respect to age and glycemic control, forced vital capacity (%predicted) of patients aged ≥60 years old with poor glycemic control also was significantly lower than those with good glycemic control (p < 0.001).Conclusions. The pulmonary function test of the patients with diabetes indicates restrictive lungs. Poor glycemic control impairs lung function and may cause respiratory symptoms, especially in the elderly.