To clarify the indications for local fibrinolytic therapy for acute cerebral embolism, correlation among initial regional cerebral blood flow (rCBF) abnormalities, recanalization timing, and computed tomographic scan (CT) findings two days later were evaluated in 17 cases. All cases included had embolic occlusion of middle cerebral or internal carotid arteries which showed no abnormal findings on initial CT corresponding to the acute events but did show abnormal CBF reduction on initial single photon emission computed tomography (SPECT). Relative rCBF was evaluated as the percentage radioisotope counts in the region of interest (ROI) of the affected side against the corresponding ROI in the unaffected contralateral side. Within 6 hours from onset, there was a tendency towards reversed time dependent tolerance of cortical infarction with residual relative rCBF. Hemorrhagic transformation was observed in one case whose relative rCBF was 35% or less. In 10 patients whose occluded arteries were not recanalized within 6 hours, cortical areas with residual relative rCBF of 70% or more did not develop infarction. In conclusions, a pre-therapeutic rCBF study using SPECT is considered to be mandatory: cases with moderate ischemia involving the cortex with residual relative rCBF of between 35% and 70% may be good candidates for local fibrinolytic therapy.