Both hypoglycaemia and alcohol consumption affect cognitive function but it is unclear whether moderate drinking alters awareness of hypoglycaemia. We have examined this in a single blind randomised hyperinsulinaemic clamp study in eight non-diabetic subjects and seven Type 1 (insulin-dependent) diabetic patients. After 30 min of euglycaemia (blood glucose 4.5 mmol/l) subjects drank either 0.75 g/kg ethanol or a placebo drink after which blood glucose was lowered to 2.5 mmol/l for 40 min. Awareness of hypoglycaemia, reaction time and physiological responses were measured before and after ethanol. At a blood glucose concentration of 4.5 mmol/l, ethanol (producing peak blood levels of 20-25 mmol/l) caused a transient increase in systolic blood pressure (p less than 0.05), a sustained increase in heart rate (p less than 0.01) and a slowing of reaction time in both normal subjects and diabetic patients. During hypoglycaemia in both groups, the slowing of reaction time and increase in sweating were more marked after ethanol than placebo (both p less than 0.05), while the increase in finger tremor (p less than 0.05) was blunted after ethanol, in both groups. Counter regulatory hormone secretion was not affected by ethanol. Despite increases in symptoms during hypoglycaemia, only 2 of 15 individuals "felt hypoglycaemic" after ethanol compared to 11 out of 15 after placebo. We conclude that after moderate drinking non-diabetic subjects and Type 1 diabetic patients are less aware of hypoglycaemia despite exaggerated physiological changes.