Ultrasonography has been shown to be of great value in studying drusen, cupping, and elevation of the optic disk, as well as mass lesions of the orbital optic nerve. We have found A-scan ultrasonography to be an accurate and simple method of measuring both the diameter of the retrobulbar optic nerve and the size of the subdural perineural space. We have found the intervaginal space of the retrobulbar optic nerve to be uniformly enlarged in papilledema, optic nerve lesions, and endocrine orbitopathy.