Although the localization of sacular aneurysm, which are the most frequent cause of subarachnoid haemorrhage, is determined exactly with cerebral angiography, the false negative angiography rate varies from 10 to 25 %. Thrombosis. vasospasm and a number of other factors have an effect on false negative angiographic appearances. Two patients with aneurysm that could not be seen on initial cerebral angiography were taken to operation depending solely on computerized tomography and clinical findings. The causes of false negative angiographic appearances are briefly summarized in the light of the pertinent cases.