It is well known that cardiovascular morbidity and mortality are high in diabetic patients. Cardiac involvement is silent and early and these diabetic patients generally complain of chronic fatigue. This study was designed to evaluate the relation between glycemic control and exercise capacity in 330 diabetic patients who have no cardiac symptoms by sustaining dynamic exercise. After a cardiac examination, patients with coronary heart disease, ECG abnormalities, cardiac failure, valvular disease, cerebrovascular disease, peripheral artery disease, anaemia and peripheral neuropathy were excluded. Plasma HbA_1c and lipid levels were obtained and a symptom limited exercise test based on “Bruce Protocol” was performed on all patients. Plasma HbA_1c levels were significantly increased in smokers and in hypercholesterolemic patients (p<0.001, p=0.006). A moderate correlation between exercise capacity and HbA_1c levels, and a weak correlation between duration of diabetes, age, sex, hypertension and plasma lipids were obtained. Multivariant regression analys is revealed that only HbA_1c and hypercholesterolemia affected exercise capacity independently (r=-0.54 r=-0.30). In conclusion, poor glycemic control in diabetic patients causes earlier cellular involvement. Because of the high affinity of HbA_1c to oxygen, the energy metabolism of the cell is affected, with a clinical correlation betreen chronic fatigue and worsening exercise capacity.