Perfusion and diffusion-weighted magnetic resonance imaging (MRI) can demonstrate, respectively, cerebral ischemia and ischemic brain injury in the first several hours after onset of symptoms, when proton density and T2-weighted MRI may appear normal. It is hypothesized that these techniques could distinguish regions destined for infarction from those that will not progress to infarction. We provide preliminary evidence from an analysis of 19 patients with severely disabling clinical deficits attributable to ischemia in at least an entire division of the middle cerebral artery, that initial perfusion and diffusion MRI were more accurate than conventional MRI in predicting no, partial or complete improvement--17 of 19 cases (p < 0.0001) versus 10 of 19 cases, respectively. In the subset of patients studied within 6 h of onset, diffusion/perfusion MRI was an even better predictor than conventional MRI--11 of 12 versus four of 12, respectively. In this small sample of patients with severe clinical deficits, perfusion and diffusion MRI were highly accurate in distinguishing those who would improve from those who would not. These results need to be confirmed in a larger prospective study, which may support a future role in the initial screening, selection, and evaluation of patients with stroke for acute pharmacologic interventions.