In this study, we employed techntium-99m hexamethylpropylene amine oxime (Tc-99m HMPAO) lung scan to detect subclinical lung injury of patients with noninsulin-dependent diabetes mellitus (NIDDM) who had normal chest X-ray findings (CXR) and pulmonary function test (PFT). The degree of pulmonary vascular endothelium damage was represented as lung/liver uptake ratios (L/L ratios) calculated by Tc-99m HMPAO lung scan. The L/L ratios of the 20 male NIDDM patients with normal CXR and PFT were compared with those of the 20 male normal controls. The results show that the L/L ratios on Tc-99m HMPAO lung scan were significantly higher in NIDDM patients than those in normal controls. Using a cutoff value of 0.50, 17 of the 20 (85%) NIDDM patients had abnormally increased L/L ratios. Our findings concluded that the pulmonary vascular endothelium damage represented as significantly increased L/L ratios on Tc-99m HMPAO lung scan in NIDDM patients with normal CXR and PFT. In addition, Tc-99m HMPAO lung scan has the potential to be a sensitive, objective, and noninvasive method to detect subclinical lung injury of patients with NIDDN, which are different from the traditional studies such as CXR or CFT.