Seven patients with diabetes mellitus were hospitalized and their blood sugar concentrations regulated as a result of fasting blood sugar, sugar around meals, urinary sugar, and hemoglobin AIC assays. Erythrocyte half-life as measured by 51 Cr increased in all patients from a mean of 27 days to 31 days, while hemoglobin AIC levels decreased from a mean of 10.1% to 5.6%. Leukocyte adherence increased in all patients from a mean of 28% to 51%. Most striking were the changes observed in platelet function in response to epinephrine. The length of the secondary lag phase of platelet aggregation, after a stimulus with final concentration of 70 muM of epinephrine, increased from a mean of 19 seconds to 65 seconds. Studies in additional patients confirmed an inverse correlation between hemoglobin AIC concentration and the secondary lag phase (r = 0.87, P less than 0.001). These studies found that certain secondary sequelas of diabetes can be corrected by strict carbohydrate control and confirmed that hemoglobin AIC assays provide a useful means of showing the degree of control of glucose metabolism in diabetic patients.