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Background Macrovascular disease, especially cardiovascular accounts for most of the mortality in patients with type 2 diabetes mellitus. We compared oxidative stress, thermal stability and oxygen releasing capacity of hemoglobin in patients with type 2 diabetes mellitus and diabetes-associated cardiovascular disease. Methods The study was performed on 38 control subjects, 31 diabetics, 36 diabetics with cardiovascular complications, and 33 non-diabetic cardiovascular subjects. Results Enhanced oxidative stress was shown by an increased protein carbonyl content observed both in plasma and in hemolysate of the diseased samples in type 2 diabetes and diabetes-associated cardiovascular diseased patients. Altered levels of cytoprotective enzymes were shown by decreased catalase activity, increased glutathione reductase activity and unaltered superoxide dismutase activity. Peroxidative activity of diseased hemoglobin was much higher compared to healthy controls indicating possible structural changes in pathologic hemoglobin molecule as a result of disease induced oxidative stress. This result is in good agreement with the observation that thermal stability of pathologic hemoglobin was also found to be less compared to control subjects. Enhanced oxygen releasing capacity of tetrameric oxyhemoglobin was monitored in presence of the drug Trifluoperazine in pathologic red blood cells, maximum increment being noticed in diabetic cardiovascular diseased subjects. Conclusion Hyperglycemia-induced oxidative stress was responsible to affect the thermal stability and oxygen releasing capacity of hemoglobin and the effect is more pronounced in diabetes-associated cardiovascular disease.