After the institution of computed tomography (CT), 814 cases of spontaneous subarachnoid hemorrhage (SAH) were treated during a period of 6 years and 9 months (April 1978 through December 1984). In 9 (22.0%) of 41 patients whose cause of SAH was not determined by the first four-vessel study (cerebral panangiography), ruptured aneurysms were found by repeated four-vessel study. Thus, of 814 cases, only 32 (3.9%) were diagnosed as cases of unknown etiology at discharge. The 32 cases were monitored by follow-up examination for 5 to 67 months (median, 27.1 months) after onset. No recurrence of SAH was reported, and all patients were rehabilitated except 2 who suffered terminal carcinoma. A third follow-up four-vessel study was performed 8 to 44 months (median, 22.0 months) after the second study in 14 of the 16 patients with SAH initially demonstrated by CT. In 1 of these cases, an aneurysm found 9 months after the initial SAH was treated surgically. Ultimately, 31 cases (3.8%) were diagnosed as cases of SAH of unknown cause. This incidence is low when compared with those in previous reports. Because of the strict examination schedule including repeated angiography, the incidence is lower and the prognosis is relatively favorable.