The clinical and neuroradiological characteristics of cerebral thrombosis and cerebral embolism were established in 109 cases with nonembolic and 120 cases with embolic arterial occlusion of the brain. Cases with cerebral thrombosis due to occlusion of perforating branches were excluded from the study, to equalize the size of the involved arteries in the two groups. More than 60% of patients with cerebral embolism showed a level of consciousness lower than stupor, while less than 15% did so in the thrombotic group. Cortical signs such as aphasia, agnosia, conjugate deviation, etc., were more frequently seen in the embolic group. A mean size of the infarct on CT (Infarct-Index) was significantly larger, and a shift of the midline structure was more frequently noticed in the embolic group than in the thrombotic group. Accordingly, the patients with cerebral embolism showed more unfavorable outcome than those with thrombosis. Hyperdense area developed in 40% of cerebral embolism during the period of observation, while in only 2% of cerebral thrombosis. Recurrence of the attack was very common in cerebral embolism. Forty-four percent experienced more than two episodes of cerebral and/or systemic embolization; of these recurrences 51% (27% of all cases) occurred within 2 weeks before and/or after the present episode that caused the admission. The importance of atrial fibrillation as the source of emboli was reemphasized.