A natural history of ischemic cerebral edema was analyzed in 100 patients with occlusive cerebrovascular diseases admitted within one week of onset, with serial computed tomography (CT) and angiograms. Cerebral edema was defined as a low-density area with mass sign on CT. A total of 446 studies of CT were performed. In 40 patients with major completed stroke admitted 24 hours of onset, a total of 73 studies of CT were taken within 48 hours of onset, and a timing of appearance of cerebral edema was analyzed. Patients were allocated to two groups according to the findings on angiograms; 68 patients who showed occlusive lesions on sequential angiograms (the group of no-recanalization), and 32 patients who showed reopening of occluded vessels on the first or sequential angiograms (the group of no-recanalization). In the group of no-recanalization a low-density area gradually appeared between 6 and 24 hours of onset A. mass sign reached its peak on the 3rd day with a peak incidence of 56%, and midline shift was present in 30% of the cases. It was resolved within 2 weeks in most cases. In the group of recanalization, a low density area appeared after 4 hours and rapidly increase after 6 hours. All patients had a low-density area at 12 hours of onset. A mass sign progressed until the 6th day with a peak incidence of 82%, and midline shift was present in 50% of the cases. The duration of mass sign was longer than that in the group of no-recanalization. These results suggest that the permissible duration of ischemia is considered to be within 6 hours of onset in profound ischemia. After 6 hours, ischemic tissue damage may not be reversible by the revascularization, even if CT shows no change during ischemia. An abrupt revascularization may not contribute to the recovery of brain damage, but accelerate the ischemic cerebral edema.