Objective[|]In this study, the parameters of oxidative stress markers and thiol–disulfide homeostasis as a novel biomarker were evaluated in experimental groups of adult individuals, which were formed according to the body mass index (BMI).[¤]Materials and Methods[|]A total of 165 adult patients were grouped as normal weight (BMI 18.5–24.9, n=39), preobese or overweight (BMI 25–29.9, n=47), obese (BMI 30–34.9; n=44), and severely obese (BMI >35, n=35). In addition to thiol–disulfide homeostasis parameters, the total oxidant status (TOS), total antioxidant status (TAS), oxidative stress index (OSI), ischemia-modified albumin (IMA), albumin, and ceruloplasmin levels were determined.[¤]Results[|]Native thiol, total thiol, and native thiol/total thiol % levels were significantly decreased in the overweight, obese, and severely obese groups compared to the normal weight group (p<0.001). Disulfide levels were elevated in the overweight group compared to the normal weight group (p<0.01). While the TOS and OSI levels of the normal weight group were elevated compared to the overweight (p<0.001) and obese/severely obese groups (p<0.05), albumin levels of the normal weight group were reduced compared to other groups (p<0.001). The IMA levels of the overweight group were elevated compared to the normal weight and severely obese groups (p<0.05 and p<0.001, respectively). Ceruloplasmin levels of the severely obese group were increased compared to the normal weight and overweight groups (p<0.001 and p<0.01, respectively).[¤]Conclusion[|]In our study, oxidative stress was increased in groups with a BMI greater than normal (≥25). In addition to this, the oxidative stress and thiol–disulfide homeostasis markers are observed to be further increased in the overweight group than the obese (≥30) group due to body’s reaction to first inconsistency.[¤]