Control of diabetes in a group of 82 insulin-treated diabetics was assessed by in-patient 24-hour plasma glucose profiles and haemoglobin A1 (HbA1) estimation. Thirty-two of these patients (39 per cent) had hypoglycaemia (plasma glucose less than or equal to 2 mmol/l) which was rarely associated with symptoms. Twenty-seven (61 per cent) of 44 patients who took a series of out-patient pre-prandial capillary blood samples over a three-day period had hypoglycaemia. Conventional measurements of diabetic control including fasting plasma glucose and HbA1, were lower in patients with hypoglycaemia than in those without. Rebound hyperglycaemia following hypoglycaemia 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 hypoglycaemia, consistent with the absence or rebound hyperglycaemia. Diabetic retinopathy was less prevalent in patients with hypoglycaemia, 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 hypoglycaemia.