In this study, we evaluated the efficacy of postoperative intrathecal injections of tissue-type plasminogen activator (tPA) in preventing cerebral vasospasm in cases with a diffuse severe subarachnoid hemorrhage. All 10 cases were graded Group 3 according to the classification of Fisher and associates, and the CT number (Hounsfield number) of the subarachnoid clot was over 75. After clipping the aneurysm and removing the clot, three cisternal drainage catheters were inserted into both sylvian cisterns and the prepontine cistern, and continuous ventricular drainage was performed routinely. Postoperatively, tPA (0.5 mg/2.5 ml) was infused as a bolus into both basal cisterns and the lateral ventricle twice daily for about 6 days. Angiography and cerebral blood flow studies using single photon emission computed tomography were performed on Day 4 or 5 and between Days 7 and 10 after onset of the hemorrhage. To date, there have been no cases that have shown angiographic vasospasm or delayed ischemic neurological deficits. This preliminary study indicates that the intrathecal bolus injection of tPA produces a marked effect on vasospasm.