OBJECTIVE: Chronic obstructive pulmonary disease (COPD), asthma and asthma-COPD overlap syndrome (ACOS) are obstructive pulmonary disorders with different manifestations. Status of oxidation in tissues is important in obstructive pulmonary disorders. Smoking, acute exacerbations of COPD and asthma were associated with a marked imbalance in oxidant or antioxidant status due to increased oxidative stress in tissues and blood. Oxidative conditions may cause a reversible formation of mixed disulphides among protein thiol groups. The aim of this study was to compare parameters related with thiol/disulphide homeostasis in patients with COPD, asthma and ACOS. PATIENTS AND METHODS: Patients (n= 135, 69 females, 66 males) who were referred with a diagnosis of COPD, asthma or ACOS were included in the study. Thiol/disulphide homeosta-sis parameters in blood were analysed by a newly established method that measures the exact thiol/disulphide status in the body. RESULTS: The patients with COPD, asthma or ACOS were similar for demographic parameters other than age and number of cigarettes smoked. Measured thiol/disulphide homeostasis parameters were similar among these patient groups. When these biochemical measurements were adjusted for age and number of cigarettes by using regression analysis, similarity for thi-ol/disulphide homeostasis parameters among patient groups persisted. CONCLUSIONS: To best of our knowledge, this is the first study to compare thiol/disulphide homeostasis parameters in COPD, asthma and ACOS patients. Similarity of thiol/disulphide homeostasis parameters among these patient groups supports the current view of Dutch hypothesis that COPD, asthma and ACOS share similar pathophysiological features but display different clinical manifestations.