The therapeutic usefulness of hyperbaric oxygenation in cerebral vascular disease was evaluated in 22 persons with recent neurologic deficits caused by cerebral embolism, thrombosis, hemorrhage, or arteriographic complications. Hyperoxygenation produced a significant elevation in content and tension of oxygen in blood and increased the reservoir of oxygen available for utilization by neurons. Remarkable and dramatic improvement in neurologic function occurred in four patients. The improvement persisted in two patients. In two others the neurologic deficit recurred a few hours after removal from the hyperbaric chamber; repeated exposure to high oxygen pressures was associated with only temporary improvement. In six other patients there was some evidence of clinical recovery immediately after onset of hyperoxygenation, but the neurologic deficit returned during decompression. The remaining 12 patients did not improve during hyperbaric oxygenation. These observations indicate that in some patients neuronal structures remain viable for some hours after loss of function in acute cerebral ischemia. In such instances an increase in oxygen delivery may reverse cellular ischemia and prevent death of cerebral tissues. Hyperbaric oxygenation may provide supportive therapy in some patients with acute cerebral ischemia, thereby permitting the removal of the occlusive lesion by surgery or other methods.