The relationship between cisternal high density calculated by Hounsfield number (HN) in computed tomography and the subsequent development of cerebral infarction due to delayed vasospasm was studied retrospectively in 36 cases of ruptured cerebral aneurysm. All patients were hospitalized within 24 hours and underwent operation within 48 hours after subarachnoid hemorrhage. Extensive removal of cisternal blood clots was carried out after obliteration of the aneurysm. The patients were divided into two groups according to the occurrence of vasospasm. In the group without vasospasm, the average HN was 65.7 preoperatively; after operation, the HN declined significantly to 62.2. In the group with vasospasm, the average HN was 77.6 preoperatively and 77.5 postoperatively. The relation between HN in postoperative computed tomography and vasospasm was as follows. When HN was 68 or less, vasospasm did not occur in any case. When HN ranged from 68 to 73, the incidence of vasospasm was 50%. When HN was 73 or more, vasospasm occurred in all cases. We propose a new system using HN to predict the incidence of vasospasm. This system is useful in deciding the timing of operation and determining the amount and location of clots to be removed.