A case of unilateral visual field defect due to optic nerve compression by non-sclerotic internal carotid and ophthalmic arteries was reported. A 14-year-old boy was admitted to our department because of constricted visual field of the right eye. The right ophthalmic artery originated from more distal portion of the internal carotid artery than usual on angiogram. CT, PEG and other roentgenologic examinations revealed no abnormalities. But the possibility of para-sellar tumor still remained, so the right optic nerve was explored by a right frontotemporal approach. The optic nerve was found to be compressed not by any sort of mass lesion but by the non-sclerotic internal carotid and ophthalmic arteries against the rim of the optic canal. Then a microsurgical unroofing of the optic canal was performed to relieve the nerve from the compression. The decompressed upper surface of the nerve was found to be discolored yellow-white. Unfortunately, no improvement in the visual field was obtained after operation. Visual field defect due to vascular compression of the optic nerve has been discussed in only several reports. There have been only 8 cases of surgically treated unilateral visual field defect due to vascular compression. Most of them were the results of compression by the sclerotic or fusiformly enlarged internal carotid arteries and were found in the patients of middle age. Four cases presented with lower half visual field defects.(ABSTRACT TRUNCATED AT 250 WORDS)