The hypoglycemic effect of alcohol, probably caused by inhibition of gluconeogenesis in the liver, is well known. However, accurate data on the effect of usual amounts of alcohol on the blood sugar in type 1 diabetics are not available. We therefore investigated the effect of an evening dose of alcohol on the blood sugar in 23 male type 1 diabetics (age 32.8 [20 to 64] years, duration of diabetes 11.5 [0.5 to 38] years, Broca index 0.98 [0.83 to 1.34] HbAlc 8.4 [5.8 to 12.2]%). On one day, the diabetics were given 0.56 g ethanol per kg body weight to drink in the evening, corresponding to about one liter of diabetic beer, on another day, the same volume of mineral water. In addition, eleven of the 23 patients received half the amount of alcohol on a third evening. Between 7 and 11 o'clock in the morning after alcohol consumption, blood glucose levels were significantly lower. The number of hypoglycemic episodes in the morning of the day following alcohol administration was appreciably greater. Usual amounts of alcohol consumed after the evening meal thus increase the risk of hypoglycemia the following morning. Diabetics should be instructed accordingly.