Two cases of ruptured blood blister-like internal carotid artery aneurysms for which low flow bypass was sufficient to attain successful treatment of trapping are reported. In the acute stage of rupture, it is troublesome to perform accurate examinations of tolerance to ischemia like balloon occlusion test(BOT)for estimating the required amount of bypass flow. In our cases, X-ray angiography perfusion(XAP)analysis was introduced, which could be performed in a couple dozen seconds without room-to-room transfer of patients, following the ordinary examination of diagnostic digital subtraction angiography. The perfusion index(PI)ratio measured in this analysis is equivalent to the laterality of cerebral blood flow between the right and left hemispheres. The PI ratio of 0.85 approximately corresponds to the mean stump pressure(MSTP)of 40mmHg, on the basis of the correlation diagram between the PI ratio and MSTP(approximate straight line:PI ratio%=0.6×MSTP+60). Even though the PI ratio of the cases was superior to this threshold of tolerance for parent artery occlusion, complementary low flow bypass was added in the acute case for the overwhelming succeeding vasospasm and for securing the flow to peripheral perforators, which resulted in a successful treatment without any ischemic events.