During a 7-year period, we observed 58 patients with signs and symptoms of subarachnoid hemorrhage (SAH) in whom clinical and neuroradiological investigations failed to reveal a reasonable cause of the bleeding. Repeat panangiography was negative in the 2 patients with spasm. Rebleeding episodes soon after admission were rare, and the overall rebleeding rate was 3.4% (equivalent to an annual recurrence of 0.6%). In 1 case, a second extensive angiographic evaluation showed a small cerebral arteriovenous malformation, which was successfully treated surgically. On follow-up, 89% of the patients experienced a good outcome, with return to normal activities. This study confirms a good prognosis for patients with SAH of unknown cause. The necessity of performing a second angiographic study and the role of small infundibular dilations of the cerebral arteries are discussed.