When the ability of granulocytes from 10 poorly controlled diabetic patients with fasting hyperglycemia and no evidence of ketoacidosis (mean fasting glucose 293 ± 20 mg. per 100 ml.; mean ± S.E.M.) to adhere to a nylon fiber column was assessed, the number of adherent granulocytes from whole blood was only 53 ± 6 per cent of the values observed in controls. After antidiabetic treatment for one to two weeks and lowering of fasting glucose levels (mean 198 ± 29 mg. per 100 ml.), adherence improved significantly (p <0.01) in the diabetics; however, their values were still subnormal (diabetic 74 per cent ± 8 of control; p < 0.02). Adherence values before and after treatment correlated with the fasting glucose level (r = 0.88, p <0.001). These findings suggest that, in addition to previously reported abnormalities in migration and the ingestion and killing of bacteria, granulocyte adherence may also be impaired in poorly controlled diabetic patients. This functional abnormality correlates directly with the fasting glucose and is reversed by insulin treatment. A defect of this type may compromise the normal inflammatory response in some diabetics and impair their capacity to resist infection.