Single-photon emission computed tomography with thallium-201-labeled diethyldithiocarbamate was performed in 26 consecutive patients less than or equal to 24 hours after a supratentorial brain infarction. Computed tomography excluded other relevant pathology. Two observers assessed the initial regional cerebral blood flow deficit using a semiquantitative visual method. The size of the initial flow deficit correlated negatively with the Barthel Index score (performance of the activities of daily living) after 6 months but not with the motricity index score (a measure of severity of the hemiparesis) after 6 months. The initial flow deficit was strongly correlated with early death from tentorial herniation; all five patients with the largest initial flow deficits died less than or equal to 6 days after the stroke, but among the 21 patients with smaller initial flow deficits only one died early (p = 0.00018, Fisher's exact two-tailed test). Although depressed level of consciousness, conjugate gaze deviation, and severe hemiparesis were more prevalent in the group of six patients who died early, the differences in prevalence between this group and the 20 patients who survived were not significant. Single-photon emission computed tomography is a relatively simple method to semiquantitatively measure a blood flow disturbance immediately after its occurrence. The magnitude of this blood flow disturbance is strongly related to early death from tentorial herniation. Because of these properties, single-photon emission computed tomography to measure regional cerebral blood flow can be of great use in acute stroke research.