Prolonged hypoglycaemia (serum glucose levels of 50 mg/dl and less, for more than 12 h in spite of treatment with periodic injections of hypertonic glucose) secondary to treatment with glibenclamide was found in 13 hospitalized patients. The mean daily dose of glibenclamide was 6.7 mg. In nine patients, the hypoglycaemia developed within 7 days of treatment. In two patients the tendency to hypoglycaemia lasted for more than 60 h in spite of continuous infusion of 5% or 10% glucose. Old age seems to be a crucial predisposing factor as none of the patients was under the age of 68 years. Contributing factors were renal failure and congestive heart disease. We feel that glibenclamide should be used with care in the elderly and in patients with renal or cardiac failure.