In 52 cadaveric half-heads, an endoscopic sphenoethmoidectomy was performed. Then, an anatomical preparation with registration of specific data like bulging of the optic canal, thickness of the bony wall covering the optic nerve and the internal carotid artery was achieved, followed by histological sections in specially selected cases. Thus the two key areas of major surgical hazard in the posterior rhinobasis could be clearly demonstrated: the immediate topographic relation of the optic canal and the internal carotid artery to the lateral wall of the sphenoid sinus and the cells of the posterior ethmoid respectively. Onodi cells of varying degrees were found in 42% of all cases. The thickness of bony wall over the maximum bulging of the optic canal averaged 0.28 mm. Bony dehiscences could be demonstrated in 12% of the cases. The technique of data acquisition, the anatomical and histological findings as well as their clinical and surgical relevance are discussed.