Regional cerebral blood flow, fractional oxygen extraction and oxygen metabolism have been measured in 34 patients after acute nonhaemorrhagic cerebral hemispheric infarction. Nine cases showed elevated oxygen extraction in the region of the early infarct, and these were the patients studied earliest after the onset of stroke. The results of serial studies to follow the evolution of the pathophysiology of acute stroke in these 9 patients are presented. The elevated oxygen extraction within the early infarct showed a significant reduction over the week following the onset of stroke. The reason for this fall in the fractional use of available oxygen varied in individual cases, and at the extremes was associated with a marked reduction in oxygen metabolism with a further small fall in residual blood flow, or a return of flow without recovery of oxygen metabolism. The significance of oxygen extraction in terms of potential viability of the tissue is discussed. The finding of a lower oxygen extraction in subcortical grey and white matter compared to cortex within the first hours or days of a major stroke is considered indicative of an earlier change from ischaemia to infarction in the deep tissues, probably related to the anatomy of the microvasculature. The interpretation of the results in the light of knowledge accumulated from studies of ischaemia in animals is presented, and problems imposed on data analysis by current limitations in positron emission tomography are discussed.