The Occurrence of abnormal carbohydrate metabolism have been reported recently in patients administered with nicotinic acid in large doses for prolonged duration as a decholesterolic agent. We have studied on carbohydrate metabolism in patients with various diseases administered with large doses of nicotinic acid for a long time. Aggravation of diabetes mellitus and relatively frequent incidence of abnormal carbohydrate metabolism were observed in patients with normal carbohydrate metabolism. Occurrence of abnormality in liver function was also noted. Therefore, frequent examinations for carbohydrate metabolism and for liver function should be performed when the patients are administered with large doses of nicotinic acid for prolonged duration.