Erythrocyte half-life (Et-1/2) has been measured in 11 patients with poorly controlled blood sugar levels and compared with a normal control group to determine whether decreased red cell deformability, which occurs in diabetic patients, causes shortening of erythrocyte life or not. No difference was seen (Et-1/2 = 28.1 days in diabetic patients and 28.5 days in controls). There was also no correlation between Et-1/2 and glycosylated hemoglobin (HbA1) levels. It has been concluded that poorly controlled diabetes mellitus has no effect upon erythrocyte life.