Endothelial function and T-lymphocyte subsets in patients with overlap syndrome of chronic obstructive pulmonary disease and obstructive sleep apnea Background: The coexistence of chronic obstructive pulmonary disease (COPD) and obstructive sleep apnea (OSA) is termed overlap syndrome (OS). COPD and OSA both have increased risks of developing cardiovascular diseases. This study aimed to explore if patients with OS exhibited a higher prevalence of cardiovascular complications, and if patients with OS exhibited vascular endothelial dysfunction and abnormalities in the cellular immune function of T lymphocytes. Methods: Totally 25 patients with stable COPD (COPD group), 25 patients with OSA (OSA group), 25 patients with OS (OS group), and 20 healthy adults (control group) were enrolled between January 2017 and December 2017 from the Respiratory Department of Tianjin Medical University General Hospital. The clinical characteristics of the four groups were collected and the expression levels of soluble vascular cell adhesion molecule-1 (sVCAM-1), tumor necrosis factor-α (TNF-α), and T-lymphocyte subsets were detected. One-way analysis of variance, χ2 test and Pearson correlation were used to manage the data. Results: The prevalence of hypertension and coronary heart disease was significantly higher in the OS group than in the control, OSA, and COPD groups (χ2 = 20.69, P < 0.05 and χ2 = 11.03, P < 0.05, respectively). The levels of sVCAM-1 and TNF-α were significantly higher in the OS group than in other groups (F = 127.40, P < 0.05 and F = 846.77, P < 0.05, respectively). The percentage of CD4+ lymphocytes and CD4+/CD8+ were both significantly lower in the OS group than in any other group (F = 25.40, P < 0.05 and F = 75.08, P < 0.05, respectively). There were significantly negative correlations in the levels of sVCAM-1 and TNF-α with CD4+/CD8+ lymphocytes (r = –0.77, P < 0.05 and r = –0.83, P < 0.05, respectively). Conclusions: The prevalence of hypertension and coronary heart disease was higher in patients with OS than in patients with either OSA or COPD alone. Patients with OS exhibited more severe vascular endothelial injury, stronger inflammatory response, and lower cellular immune function.