The direct neck clipping for carotid-ophthalmic aneurysm is hazardous and difficult as compared with other intracranial aneurysms because of its location adjacent to the anterior clinoid process, the optic nerve, and the cavernous sinus. The authors discuss the visual abnormalities accompanied by carotid-ophthalmic aneurysm on the basis of our experiences with 30 patients with 33 aneurysms. Ten patients (4 of 14 with ruptured aneurysms, 6 of 16 intact aneurysms) had preoperative visual abnormalities. Of our series of patients, visual acuity and visual field were impaired in one eye. The most common field abnormalities were unilateral inner or upper side defects. In the patients whose aneurysms were intact, visual abnormalities had been slowly progressive over many months. Otherwise, 2 cases with ruptured aneurysm experienced their first visual symptoms at the same time their hemorrhage. According to the projection of aneurysm with respect to internal carotid artery, superomedial and posteromedial projections were common in the patients with visual abnormalities. Seven patients with visual dysfunctions were operated on. Improvement of the visual symptoms was observed in 3 cases. In these cases, complete collapse of the aneurysm with good decompression of the optic nerve was obtained at operation. Two patients deteriorated following surgery; this occurred in relation to attempts to clip the aneurysm neck directly, and optic nerve was probably excessively manipulated. Two patients had no change in visual symptom: one was treated with trapping of internal carotid artery combined with STA-MCA anastomosis, another had direct hemorrhage into the ipsilateral optic nerve.(ABSTRACT TRUNCATED AT 250 WORDS)