Most cases of non-traumatic subarachnoid hemorrhage are caused by rupture of an arterial aneurysm or an arteriovenous malformation. The first angiogram performed is normal nearly 20% of the time. To review the need to repeat angiography if the first study is negative. Review of patients admitted to our hospital with subarachnoid hemorrhage in recent years. All had had high quality angiography performed, that was repeated if the first study was negative. We analyzed 112 cases of subarachnoid hemorrhage, of which 27 (24.1%) had angiographic studies of the four main cerebral vessels that failed to reveal the origin of the hemorrhage. An aneurysm that had not previously been seen was detected in 2 patients (7.4%), whether or not there had been vasospasm in the first study. Given our findings, the low incidence of complications in the first study and the seriousness of the pathology under consideration, we suggest that angiography should be repeated unless there are major contraindications.