Not infrequently, cerebral angiography performed immediately after the onset of subarachnoid hemorrhage (SAH) fails to reveal any causative lesion. Although the cause of the SAH in most of these cases remains unknown, repeat angiography sometimes discloses a lesion not detected by the initial angiography. The frequency of finding a ruptured aneurysm by repeat angiography and the angiographic characteristics of such aneurysms have been investigated retrospectively. Nineteen among 316 non-traumatic SAH patients (6%) showed initially negative angiogram, and 7 of 19 patients (36%) were identified as harboring an "initially occult" aneurysm. It was possible to identify the expected cause of these initial false-negative angiograms in four of those seven patients. The aneurysm was located in the anterior communicating artery (ACoA) complex in four of the seven patients. When interpreting the angiograms of patients with SAH, neurologists and neurosurgeons should bear in mind the finding that the ACoA complex is the most frequent site of "initially occult" aneurysms. One patient had a dissecting aneurysm of the internal carotid artery, and although such lesions have been considered rare, they should be included in the differential diagnosis of SAH of unknown origin.