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.