The aim of this prospective study, carried out in a consecutive series of 305 microsurgically clipped aneurysms, was to check the absence of an aneurysmal remnant on post-operative angiography, and if a remnant was found to quantify its size in order to consider additional clipping to avoid the risk of rebleeding. Out of the 305 aneurysms, 292 (96%) were located in the anterior and 13 (4%) in the posterior circulation. Post-operative angiography was performed on average two weeks after surgery. Determination of the presence or not of an aneurysmal remnant and its quantification was done by an independent observer (JCA). Aneurysmal remnants were classified into 5 grades: grade I: less than 50% of neck size, grade II: more than 50% of neck size, grade III: residual lobe of a multilobulated sac, grade IV: residual sac of less than 75% of aneurysmal size and grade V: residual sac of more than 75% of aneurysmal size. Correlations between presence (and size) of the remnant and anatomical-surgical data obtained from the operative report were studied. Clipping was considered incomplete in 18 of the 305 aneurysms (5.9%). The group with residual neck only (grade I = 8 cases, Grade II = 4 cases) amounted to 4% of the whole series, whereas the group with residual neck + sac (grade III = 4, grade IV = 1, Grade V = 1) to 1.9%. Only this latter group was amenable to re-operation for complementary clipping without creating a stenosis of the parent artery. Our results are in the range of those of other published series. Anatomical-surgical factors for predisposition to incomplete clipping are discussed. The rates of sac obliteration using microsurgical clipping are to be compared with those recently achieved by electrically detachable coiling. The classification which we have developed is proposed for future comparison with endovascular results.