The knowledge of symptoms of hypoglycaemia of 45 consecutive elderly diabetic patients was assessed during a structured interview and compared with that of an age-matched non-diabetic control group. Twenty-three (88%) patients taking oral hypoglycaemic agents (OHAs) and six (32%) insulin-treated patients denied any knowledge of hypoglycaemia. There was no significant difference in knowledge of 14 symptoms of hypoglycaemia between the non-diabetic controls and the combined diabetic (OHA and insulin) groups, but insulin-treated patients were slightly more knowledgeable than patients taking OHAs. Fourteen (54%) of the OHA group were taking either glibenclamide or chlorpropamide which have been associated with severe and prolonged hypoglycaemia. Elderly diabetic patients are unlikely to respond to the warning symptoms of hypoglycaemia and caution should therefore be exercised when prescribing for these patients.