The value of measuring asymmetry of cerebral perfusion semiquantitatively by single-photon emission computed tomography (SPECT) correlated with measuring the volume of infarction on computed tomography (CT) was evaluated in cerebral embolism. Eighteen patients with acute cerebral embolism (mean age: sixty-nine years) were evaluated. Ten were diagnosed as having middle cerebral artery (MCA) occlusion, 3 as having MCA branch occlusion, and 4 as having unilateral and 1 as having bilateral internal carotid occlusion. The infarct volume was measured, summing up the area of infarction on CT, at 8.5 days, mean time after onset. Outcomes of the patients were classified into three groups: good, fair, or dead, judged by the consequences one month after onset. Regional cerebral blood flow (rCBF) was measured in 9 of 10 surviving patients at one month, mean time after onset, by 123[iodine N-isopropyl-p-iodoamphetamine (IMP) SPECT. Semiquantitative rCBF index of asymmetry (AI) was evaluated from four regions of brain cortex. Eleven patients with < 300 mL infarct and 7 with > or = 300 mL infarct showed a significant difference of outcome with infarct volumes (P < 0.01). The mean AI value in patients with < 150 mL infarct was 31% while it was 52% with > or = 150 mL infarct (P < 0.039). There were significant linear correlations between mean AI values and the volumes of infarction or infarct/brain volume ratios (P < 0.034 or P < 0.018). The significant correlations of the AI values with the volumes of infarction suggest that the measured asymmetry of perfusion evaluated by SPECT could reflect the ultimate tissue damage and residual intact brain volume after cerebral embolism.