Our study shows for the first time that levels of SIRT1 activity are decreased in PBMCs of patients with COPD, while remaining comparable between nonsmoking and smoking control groups; instead, SIRT1 protein expression levels were not discriminative for diseased subjects, as they were already significantly decreased in smoker versus nonsmoker controls and becoming further reduced in COPD samples. Here, SIRT1 is linked to chronic obstructive pulmonary disease.