Control of diabetes in a group of 82 insulin-treated diabetics was assessed by inpatient 24 h plasma glucose profiles and HbA1 estimation. Of these patients, 32 (39%) had hypoglycemia (plasma glucose .ltoreq. 2 mmol/l) which was rarely associated with symptoms. Of 44 patients who took a series of outpatient preprandial capillary blood samples over a 3-day period, 27 (61%) had hypoglycemia. Conventional measurements of diabetic control, including fasting plasma glucose and HbA1, were lower in patients with hypoglycemia than in those without. Rebound hyperglycemia following hypoglycemia was not seen and its absence was not due to diabetic autonomic neuropathy. Cortisol/creatinine ratios in early morning urine samples were similar in patients with and without nocturnal hypoglycemia, consistent with the absence of rebound hyperglycemia. Diabetic retinopathy was less prevalent in patients with hypoglycemia, possibly reflecting better long-term diabetic control in this group. HbA1 concentration reflects overall blood glucose control in diabetes but near-normal levels must be interpreted with caution since they may be associated with recurrent hypoglycemia.