BACKGROUND A reduction in lung capacity has been reported previously among diabetics. According to WHO estimates, Pakistan is currently eighth in the prevalence of diabetes mellitus (DM) and will become fourth by the year 2025 with over 15 million individuals. This study was designed to see the impairment of lung functions on spirometry in DM patients. OBJECTIVE Our aim was to investigate the pulmonary functions tests of Pakistani patients with DM. METHODS Between January to July 2004, 128 subjects who were never-smokers and had no acute or chronic pulmonary disease were recruited. Sixty-four of these subjects had DM and 64 were healthy matched controls. All underwent screening with detailed history, anthropometry, lipid profile, and spirometric measurements at the Aga Khan University Hospital, Karachi, Pakistan. RESULTS The mean age of diabetics and matched control were 54.3±9 and 54.0±8 (P<0.87) years, respectively. Diabetes patients showed a significant reduction in the forced vital capacity (FVC) [mean difference (95% CI) - 0.36 (-0.64, -0.07) P<0.01], forced expiratory volume in one second (FEV(1)) [- 0.25(-0.50, -0.003) P<0.04], and slow vital capacity (SVC) [- 0.28(-0.54, -0.01) P<0.04], relative to nondiabetic controls. There was no significant difference noted in the forced expiratory ratio and maximum mid-expiratory flow between the groups. There was also a significant higher level of triglycerides noted among diabetics (P<0.001). CONCLUSIONS Diabetic patients showed impaired lung function independent of smoking. This reduced lung function is likely to be a chronic complication of diabetes mellitus.