The clinical, operative and pathological characteristics of intracranial dissecting aneurysms are presented. A review of the literature suggests that these types of intracranial aneurysms are being recognized with increasing frequency and can be characterized by its symp-tomatology and radiological patterns. For this five years, 31 cases with intracranial dissecting aneurysm were treated in our institute. The mean age and the site of aneurysm are assumed almost the same as that of previous reports, but the incidence of extravascular bleeding is 58% in our series, which may be higher than that of previous reports. Although the cause or mechanism of intracranial dissecting aneurysm has yet to be sufficient clarified and it largely depends upon future multilateral studies, but we would like to emphasize that the diagnosis is most important in this disease. It has been considered that“double lumen”on angiogram is a true diagnostic sign of dissecting aneurysm. Only four cases of 31 showed“double lumen”, but no one showed it in acute stage. 17 cases (55%) showed“string and pearl”sign and 72% of the cases with ruptured dissecting aneurysm revealed it in acute stage. From these results, it seems to be quite all right to consider that“string and pearl”sign should be the true diagnostic sign in acute stage like as“double lumen”in chronic stage. We have obtained some interesting information through detailed examinations about surgical treatment of intracranial dissecting aneurysms. But we will mention it in detail on another occasion. Finally, we will conclude by listing the above-mentioned important points of the investigation about the di-agonosis of intracranial dissecting aneurysms.