Aneurysms arising from the dorsal wall of the internal carotid artery are rare. The authors surgically treated twenty dorsal internal carotid artery aneurysms. Pre-operative angiographic findings were reviewed and classified into three types. Eleven aneurysms projecting superiorly on the lateral angiogram were found to be adherent to the base of the frontal lobe by the pterional approach. Five aneurysms which had been superimposed with the internal carotid artery on the lateral angiogram were found adhered to the medial surface of the temporal lobe. Four aneurysms not seen on the angiogram had no adhesion. Two aneurysms, which had not been seen on the initial angiograms, were visualized on the angiograms taken during the period of vasospasm. This type of aneurysms can be the source of a subarachnoid haemorrhage of unknown origin and requires repeated examinations. Premature rupture occurred intra-operatively in five cases and postoperative bleeding was encountered in two. Clipping technique is discussed from the viewpoint of preventing intra- and postoperative rupture.