OBJECTIVE: Most cases of spontaneous subarachnoid haemorrhage (SAH) are due to a ruptured cerebral aneurysm, yet sometimes the cause of bleeding can be obscure. We report here on the results of a retrospective single-center study to determine the role of the hemorrhage patterns for the patients with a negative angiogram on admission. METHODS : A total of 480 patients with spontaneous subarachnoid hemorrhage (SAH) were admitted from January 2004 to September 2008. Of these, 44 patients were included in this study because of their negative findings on their initial angiography. SAH was diagnosed by a computed tomographic scan or lumbar puncture. The clinical grade was assessed using the Hunt-Hess grading system, Fisher's grading system and the modified Rankin scale. RESULT : The overall incidence of an initially negative angiogram for patients with a spontaneous SAH was 10.2%. Based on the hemorrhage pattern on the admission CT, the most common pattern was the diffuse type (52.3%), followed by the perimesencephalic type (29.5%), the CT negative type (11.4%) and the localized non-perimesencephalic type (6.8%). The repeat angiogram revealed an aneurysm in 3 (7.7%) patients and exploration revealed a dissecting aneruysm of the vertebral artery in 1 patient. The patients with the diffuse type SAH significantly differed from the perimesencephalic group with regard to the Fisher grade (p = 0.002), the outcome at discharge (p = 0.004) and the need for EVD. CONCLUSION : Patients with SAH of an unknown cause, especially with perimesencephalic SAH, have an excellent prognosis and low mortality. We believe that digital subtraction angiogram is still the gold standard for making the diagnosis of aneurysm in patients with spontaneous SAH. A repeat angiogram is recommended for all the patients with initially angionegative SAH. For those patients with perimesencephalic SAH and a positive CSF study with a negative CT, we suggest to perform a CT angiogram as a less invasive follow-up study.