We studied prospectively the syndrome of brain edema after a large infarction in 12 patients. The major symptom was drowsiness, which began on the first to fourth day after the ictus and which was accompanied by asymmetry in pupillary size of 0.5 to 2.0 mm in eight patients, periodic breathing in seven, and Babinski's sign contralateral to the hemiparesis in five. These accompanying signs appeared several hours after drowsiness in some patients. Seven patients had brain death, one died of sepsis after recovering from brain swelling, and only four survived. In six patients in whom intracranial pressure was continuously measured, levels persistently above 15 mm Hg were associated with eventual brain death (four patients) and levels below 15 mm Hg were associated with survival (two patients).