Initial results in over 50 patients with stroke suggest that positron images made during continuous inhalation of carbon dioxide labeled with oxygen 15 and molecular oxygen labeled with oxygen 15 provide data on tissue function that may be relevant to acute stroke management. Five cases illustrate the following findings: 15O-activity patterns observed in areas of ischemic injury or infarction are what one would expect if the 15O distributions represented physiologic functions, such as cerebral blood flow and metabolism. Areas of abnormal 15O activity correlate with the clinical or computed tomographic (CT) localization of the deficit. In studies performed acutely, changes in 15O distributions anticipate alterations in CT scans and may be predictive of outcome. Data related to oxygen metabolism correlate better with tissue viability than do those reflecting cerebral blood flow.