Dissecting aneurysm of the intracranial arteries is a well known clinical entity, and its angiographic findings are also well recognized. We encountered a case with dissecting aneurysm of the vertebral artery presented with subarachnoid hemorrhage (SAH). The initial angiography was normal but repeated angiography demonstrated a dissecting aneurysm. This case is reported here, and the relevant literature is reviewed. A 46-year-old woman had been well until she complained of headache. She was admitted to a local hospital and found to have SAH. She was transferred to our clinic for further examination. On admission the patient was drowsy but able to be aroused. Her neurological state was normal except for a mildly stiff neck. Computed tomography (CT) demonstrated massive SAH in the basal cistern and intraventricular hemorrhage in all ventricles. CT also demonstrated acute hydrocephalus. Four-vessel cerebral angiography was performed using transfemoral catheterization. No definite abnormalities, except for a small aneurysm in the cavernous portion of the right internal carotid artery, were found. Because aneurysm in the cavernous portion could not cause SAH, we could not define the origin of the SAH. However, retrospectively, a slightly irregular wall of the left intracranial vertebral artery was evident. Repeated angiography performed 28 days later revealed aneurysmal dilatation of this left vertebral artery. Double density of the contrast material was found in the aneurysmal dilatation. Retention of the contrast medium was also seen in the late capillary phase. From these angiographic findings, the aneurysm was diagnosed as being a dissecting aneurysm. Direct surgical attack on the vertebral aneurysm was performed via a left suboccipital craniectomy.(ABSTRACT TRUNCATED AT 250 WORDS)