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The authors present, among 200 intracranial arterial aneurysms, 13 cases where the angiographic diagnosis of the aneurysm had been difficult. Among them, there were ten mistakes through default, and four mistakes through excess. Angiographic studies were done early, about the third day following subarachnoidal hemorrhage, so that conscious patients may have rapid surgery. The main diagnosis defaults were observed in aneurysms of the anterior communicating artery (cases 1, 2, 3, 4, 6), where the flexurae and arterial anomalies explain the diagnostic mistakes; double anterior cerebral artery, median anterior cerebral artery of Lazorthes, early origin of the frontopolar artery or of the callosomarginal artery. The aneurysms of the posterior wall of the carotid bifurcation, were hidden by this artery in the A.P. view and by the middle cerebral artery in the profile view (cases 5 and 7). Oblique views are necessary for the two localizations of these aneurysms. The middle cerebral aneurysms (case 8) may be hidden by a bony superposition. The Ziedses des Plantes subtraction method obviates this diagnostic default. Aneurysms of the peripheral branches of cerebral arteries (case 9) are filled later and slightly. Aneurysms of an artery less than 3 mm wide (posterior communicating artery, lenticular artery) are difficult to discriminate from their enlarged infundibulum (case 10). The small size of the aneurysm is not the only explanation of the difficult diagnosis. Indeed, few aneurysms are not evidenced by the first angiography, done at an early stage, even if any spasms exist. Only repeated angiographies could show the aneurysm particularly in young people. We repeat the angiography 15 to 20 days after the first one, and even some months later. In two young patients, angiographic studies were repeated a few times in three years and were normal; but after four years in one case and five years in the other, a new subarachnoidal hemorrhage occurred, and only then was the aneurysm shown by a new angiography (cases 4 and 5). In our series, the angiographic spasm could not afford an explanation for the diagnostic default. This, soon after the subarachnoidal hemorrhage, might be explained by the hemostatic clot which compresses or plugs the aneurysm. Further, the clot's lysis occurs, and the aneurysm can be evident. A double conlcusion must be drawn: --first, the angiography must be repeated if the image is dubious; the angiographic technique must be perfect and varied (oblique view, subtraction method, enlargement technique). Mistakes through excess, leading to useless operations, are as dangerous as mistakes through default; --secondly, great care should be exerted when reading and interpreting the X-ray films in the post-operative period in order to evaluate the surgical results, as well as in the preoperative period.