Twenty-three patients with an arachnoid cyst in the frontal part of the temporal bone, which produced exopthtalmos, were evaluated with CT and MRI. The majority of the patients had the peak of the bowing of the bony orbit wall in the posterior third (20/23) and only three in the middle. Arachnoid cyst are classified as small (13/23), medium (5/23) or large (5/20) size. Exopthtalmos are also classified as mild, moderate or large. CT and MRI clearly revealed the exopthtalmos, the bone remodelling process and the relationship of the orbit muscles, optic nerve and posterior bony wall of the orbit.