The effect of intrathecal tissue plasminogen activator administered after subarachnoid hemorrhage on the development of cerebral vasospasm was examined. Fourteen cases with severe subarachnoid hemorrhage (Group 3 as defined by Fisher's computed tomography criteria) underwent early surgery for aneurysm neck clipping within 2 days of subarachnoid hemorrhage and were treated with t-PA after the operation. The subarachnoid clot was eliminated dramatically after t-PA introduction in all cases. None of the patients showed symptomatic vasospasm. Eight cases (57%) showed angiographical narrowing, but this was very mild and only the distal portion of the anterior cerebral or middle cerebral artery where the subarachnoid clot had remained until late was involved. Intrathecal t-PA administration was effective in early elimination of subarachnoid clots and in preventing cerebral vasospasm after subarachnoid hemorrhage.