Background <p>The incidence of diabetes is increasing worldwide, particularly in developing countries. Diabetes is associated with abnormal lung function and decreased gas exchange. This study aimed to evaluate the results of pulmonary function tests in patients with type 2 diabetes mellitus who were referred to endocrinology clinics.</p> Methods <p>A case-control study was conducted on 60 diabetic patients, 51 prediabetic patients, and 70 nondiabetic patients aged 30 to 60 years who were referred to the endocrinology clinic. FVC, FEV1, and FEV1/FVC were measured. The pulmonary function data were compared between groups through a one-way analysis of variance and independent <i>t</i>-test.</p> Results <p>One-hundred forty-one (77.9%) males and 40 (22.1%) females were included in the study, with a mean age of 40.49 ± 7.80 years old. The FEV1 in the control group was significantly higher than in the diabetic and prediabetic groups (<i>p</i> &lt; 0.05). Analysis of spirometric variables in the control, diabetic, and prediabetic groups revealed that the FVC in the control group is much higher than in the diabetic and prediabetic groups.</p> Conclusion <p>The lung function is impaired in diabetic and prediabetic individuals compared with normal subjects in a sample of Iranian people.</p>