We investigated 22 cases with subarachnoid hemorrhage (SAH) of unknown etiology by the initial digital subtraction angiography (DSA). The computed tomography (CT) scans were obtained within 24 hours from onset. Patients were divided into two groups according to SAH distribution in CT on admission; perimesencephalic SAH (PMSAH) and non-PMSAH. The category of patients belonging to the PMSAH group was subdivided into typical or extended pattern of PMSAH. Typical PMSAH pattern of CT is defined as that having the center of clot immediately anterior to the upper brainstem with no definite extension into the anterior interhemispheric fissure (IFH) or sylvian fissure. Extended PMSAH pattern includes the extension of SAH into the anterior IHF or sylvian fissure with the center of the clot similarly located anterior to the upper brainstem. The number of patients with typical PMSAH, extended PMSAH or non-PMSAH was 2, 6, or 14, respectively. Follow-up DSA was obtained in 20 Spatients. The 2nd DSA revealed the lesions as bleeding sources in 3 patients with non-PMSAH. Eight patients further underwent the 3rd DSA, which identified bleeding sources in 3 patients with non-PMSAH. No bleeding sources were detected by serial DSA in PMSAH patients. Patients with extended PMSAH may be managed like those with typical PMSAH. The 3rd DSA is required if the 2nd DSA fails to identify the bleeding source in non-PMSAH. The 2nd DSA may be necessary in PMSAH patients because of the possible identification of bleeding sources. Optimal diagnostic protocol to confirm the bleeding sources should be established in SAH patients of unknown etiology.