Eleven patients who had unilateral insidious compression of the optic nerve but exhibited no detectable abnormality on plain skull radiographs or non-tomographic views of the optic canal were studied. Paracanalicular meningioma was proved surgically in 10. Complex-motion tomography demonstrated characteristic alterations in the bone forming the optic canal in all cases. These changes consisted of canal enlargement, changes in contour, and an atypical radiographic appearance of the cortical bone, seen as (a) diffuse, spongy, non-homogeneous thickening of the canal margins; (b) multiple tiny saw-tooth-like exostoses projecting into the canal lumen; or (c) short, well-circumscribed areas of smooth bone thickening. Other techniques such as angiography and pneumoencephalography were not as helpful in the diagnosis as complex-motion tomography.