Serial measurements of cerebral blood flow (CBF) were performed in 12 patients with acute symptoms of ischemic cerebrovascular disease. CBF was measured by xenon-133 inhalation and single photon emission computer tomography. Six patients had severe strokes and large infarcts on the CT scan. They showed in the acute phase (Days 1-3) very large low-flow areas, larger than the hypodense areas seen on the CT scan. The cerebral vasoconstrictor and vasodilator capacity was tested in the acute phase following aminophylline and acetazolamide, respectively. A preserved but reduced reactivity was seen at both tests in all 6 cases in the infarct and the peri-infarct areas. On Days 5-25, 4 of the patients had transitory increases (59-108%) of CBF, probably corresponding to lysis of an intracerebral embolic occlusion. The other 2 patients showed on Days 7-15 only a moderate CBF increase (appr. 20%), both had occlusion of the relevant internal carotid artery. In all 6 patients, CBF studies at 2 and 6 months resembled the acute phase, showing large areas with reduced flow. At the 6 months follow-up, the vasodilatory stress test was repeated, and all but one showed a preserved but reduced vasoreactivity in the infarct and peri-infarct tissue. Of the remaining 6 patients, one had a pontine infarct and one had no lesions on the CT scan, both having normal angiograms and CBF maps. Four patients had small deep or subcortical CT lesions, and showed a slight, but persistent CBF reduction of about 6-8% in the parietal region on the affected side. No changes in the flow pattern were seen at the vasoreactive studies. A likely explanation for the finding of superjacent low-flow areas is an intrahemispheric uncrossed diaschisis. This interpretation is discussed in relation to the peri-infarct low-flow area seen in the 6 cases with large infarcts.