This study aims to evaluate the significance of the changes of erythrocyte reduced glutathione (GSH) in the course of diabetes mellitus including the pre-diabetes stage and cardiovascular disease co-morbidity. A total of 222 participants (female:male, 107:115) were selected and their erythrocyte GSH levels were measured. The participants were divided into four groups: (i) control; (ii) those with blood glucose level '5.6 mmol/l but < 6.9 mmol/l as pre-diabetes mellitus with no other pathology; (iii) diabetes without co-morbidity; and (iv) those with diabetes mellitus and cardiovascular disease. Statistical analysis was by ANOVA followed by a Fisher's LSD post hoc test. We observed that GSH concentration was significantly different between groups (P < 0.04). The Fisher's post hoc test indicated significant differences in erythrocyte GSH levels between the pre-diabetes mellitus and diabetes mellitus groups compared to control (P < 0.005 and P < 0.05, respectively). A statistically significant change (P < 0.001) involving an initial fall followed by a rise in erythrocyte GSH levels was observed when diabetes mellitus and diabetes mellitus+cardiovascular disease groups were combined and assessed with respect to period of diabetes. We conclude that oxidative stress is already present in the pre-diabetes stage as determined by the fall in GSH, representing the initial phase of oxidative stress in diabetes mellitus progression. This finding provides evidence that antioxidant markers such as GSH could be a useful tool for pre-diabetes mellitus screening.