Surgery is currently used for treatment of incidental unruptured middle cerebral artery aneurysms. Intra-operative aneurysm rupture and ischemia dependent on prolonged regional circulatory interruption or inadequate placement of the definitive clip leading to vascular occlusion and post-operative brain infarction are direct causes of mortality and morbidity related to surgery. Temporary clips are used to deal with intra-operative aneurysm rupture and to facilitate aneurysm dissection, and a method of monitoring that could predict the danger for ischemia during temporary clip application and after the placement of definitive clip could be an important contribution for the safety of aneurysm surgery.In this study, the partial pressure of tissue oxygen (PtiO2) was monitored during surgery, in patients without subarachnoid hemorrhage. PtiO2and temperature probes were placed in a standardized way, in an attempt to avoid variations due to different positions of the catheters. PtiO2reflects the oxygen concentration in a small area of the brain tissue (a sensitive surface of 7.1 mm2around the probe), depending on the cerebral blood flow, under stable conditions of brain temperature.[2–48101620] Therefore, the regional circulatory interruption by the use of temporary clips leads to a decrease in PtiO2values,[2–48101620] and the same may happen if there is an incidental total or partial occlusion of branches of middle cerebral artery by the definitive clip. In this work, we try to establish values of PtiO2, during surgery, that may indicate a high risk for post-operative ischemic lesions.