We reported three cases of cerebral aneurysms hardly detectable by cerebral angiography, but easily detected by three-dimensional CT angiography (3D-CTA). These cases were ruptured aneurysms with subarachnoid hemorrhage. After detection of subarachnoid hemorrhage on CT scan, cerebral angiography was performed at first, but aneurysms were not detected. Subsequently 3D-CTA was carried out, and aneurysms were detected. In all cases, cerebral angiography was repeated, after the aneurysms had been found by 3D-CTA. This time aneurysms were all detected by cerebral angiography, but each case needed photographs from special direction. The aneurysms were small by usual cerebral angiography and they were almost invisible behind the artery near which they existed. 3D-CTA was very useful for detection of small aneurysms, but small perforating arteries around the aneurysms were invisible by 3D-CTA. To find these perforating arteries, cerebral angiography was needed.