In order to evaluate the safety of the internal carotid artery (ICA) occlusion based on our simple tolerance test, we reviewed 142 consecutive tolerance tests. Permanent endovascular ICA occlusion was performed on 99; a tolerance test only was performed on the remaining 43. Our assessment consisted of 1) angiographic evaluation of collateral circulations without and with ICA test occlusion, 2) evaluation of clinical tolerance to balloon ICA occlusion for 15-20 minutes. Complications of test and permanent ICA occlusion were retrospectively analyzed. Complications related to test occlusion occurred in 2 cases (1.4%) without causing permanent deficits. Complications related to permanent occlusion occurred in 16 cases (16%) including 2 technical, 10 temporary (10%) and 4 permanent (4%). Complications were significantly decreased after introduction of stringent postoperative care to prevent hypotension in 1987. Since 1988, we performed 47 permanent ICA occlusion and experienced no technical, 3 temporary (6%) and no permanent complications. Our method to evaluate tolerance to ICA occlusion is simple and safe. Reliability of the results is comparable to other more complicated methods of assessing ICA occlusion.