Knowledge of the symptoms of hypoglycaemia was assessed in 161 elderly diabetic patients using insulin or a sulphonylurea. Nine percent knew no symptoms. Patients on insulin and those with poorer control had significantly greater knowledge (p less than 0.01). Those reporting episodes of hypoglycaemia had significantly greater knowledge of some symptoms. Knowledge decreased with age, but was not related to sex, duration of diabetes or living alone. Patients and relatives had similar knowledge and education of both needs to be improved.