Periorbital dermoid cysts should be removed because they commonly leak their irritant contents into the surrounding tissues. The underlying bone may, however, be involved in patients with dermoid cysts at the outer canthus. Computed tomography studies of 70 patients (43 men and 27 women, aged 30 months to 63 years, mean 29 years) with proved dermoid cysts of this type were reviewed. The lesion was always unilateral; 34 were on the left. The bone of the lateral wall and superotemporal angle of the orbit showed the following abnormalities, often in combination: pressure erosion in 61 cases and an otherwise abnormal shape, probably developmental, in 55; the dermoid cyst entered a tunnel or canal through the lateral wall in 24; a blind pit or crater in 15; and a cleft in 20; many patients also showed abnormal bone texture. These findings are extremely important for planning adequate surgery, and indicate that bony involvement is much more frequent than previously appreciated.