Patients with type 2 diabetes have impaired exercise capacity. While numerous factors are known to contribute to impaired exercise capacity, the role of lung function remains unclear. We conducted the present study to investigate the influence of lung function on exercise capacity in patients with type 2 diabetes. Cardiopulmonary exercise testing was carried out in 31 male patients with type 2 diabetes without diabetic complications or cardiopulmonary diseases. Patients with abnormal spirometry results such as a percentage of predicted forced vital capacity (%FVC) < 80% and/or a ratio of forced expiratory volume in one second (FEV1) to FVC (FEV1/FVC) < 70% were excluded from the study. We used the percentage of predicted maximal oxygen uptake (%VO2max) as an index of exercise capacity. The correlations between %VO2max and lung function and other factors known to be associated with impaired exercise capacity were then assessed. Univariate analysis revealed %VO2max correlated significantly with percentage of predicted FEV1 (%FEV1), duration of type 2 diabetes, regular exercise habits, and systolic and diastolic blood pressures. In a multivariate analysis, %FEV1 and regular exercise habits were found to be independent determinants of %VO2max. A mild reduction in %FEV1, which may be a complication of diabetes, is associated with impaired exercise capacity in patients with type 2 diabetes. When evaluating spirometric values in patients with type 2 diabetes, a reduction in %FEV1 should be noted even when both %FVC and FEV1/FVC are within normal limits.