© 2020 De Gruyter. All rights reserved.Objectives: Obesity has become an important public health problem because of its increasing prevalence and relation with many diseases and mortality. Studies have shown its association with oxidative stress. In this study, the effect of obesity on total amount of thiol and some glutathione S-transferase (GST) isozymes were investigated which could serve as an important criteria in dose adjustment of some certain drugs in obese. Methods: The gastric tissues removed by gastrectomy operation from 29 morbid obese female patients were analysed for thiol levels and activities of total GST, GSTT1-1 and GSTM1-1. Patients were grouped according to age, presence of hypertension and/or diabetes, and family history. Results: The average total thiol was 131.22 (±7.74) nmol/ mg protein with no significant differences in between the groups. GSTT1 specific activities were about 20% higher in four groups: with ages over 35 years old, with hypertension, without diabetes and finally without family history, with respect to other groups. The differences between total GST and GSTM1 activity levels of experimental groups were not significant. Conclusions: This is the first study to compare activities of GST isozymes and total thiol content in the stomach tissues of obese female patients accompanying some common metabolic disorders, age and family history.