Headaches affecting 117 insulin-dependent diabetic patients were studied. 50 developed 3 varieties of headaches associated with clinical hypoglycaemic episodes: (1) Brief headaches, contemporaneous with cerebral and autonomic symptoms, were relieved within minutes of ingesting carbohydrates (8 patients). (2) Prolonged headaches outlasting hypoglycaemic symptoms by 1-48 (average 4.3) hours, not relieved by food, occurred in 36 patients; 12 of these also had nausea, vomiting or photophobia. (3) Migraine headache. 11 of the 117 patients were migraineurs: in 6 of the 11 their typical migraines (2 classical and 4 common) were induced by hypoglycaemic episodes. 9 of the 50 had 2 types of headaches, easily distinguished by each subject. In the whole series of 117 patients, 9 had never had a headache in their life. The remainder had headaches associated with premenstrual tension, anxiety, alcohol or other causes.