Abstract： Objective To investigate the cause of increased blood cell volume and its action on microangiopathy in diabetic patients.Methods Some parameters of blood cell and Na + K + ATPase and Ca 2+ ATPase activities in erythrocyte membrane were determined by automatic blood cell analyzer, modified Hanahan’s, and Luthra’s methods in 52 diabetic patients and 49 normal controls. Results Activities of Na + K + ATPase and Ca 2+ ATPase in erythrocyte membrane were decreased in the diabetic patients, especially those with microangiopathy. Mean corpuscular volume (MCV) and mean platelet volume (MPV) increased and mean corpuscular haemoglobin concentration (MCHC) decreased in the patients with microangiopathy. Whereas they were normal in those without microangiopathy. The changes of MCV, MPV and MCHC were related to the decrease in both ATPase activities in erythrocyte membrane.Conclusion The increase in blood cell volume relevant to the decrease in ATPase activities were probably important factors in the pathogenesis of diabetic microangiopathy.