Since exophthalmos is caused by any decrease in the size of bony orbit or occurrence of mass within the rigid orbit, the accurate diagnosis of its causes are essential in determining the therapeutic aims. Exophthalmos is one of the important signs being the indication for orbital computed tomography along with periorbital swelling, visual loss, orbital trauma and diplopia. CT as the diagnostic tol for the cause of exophthalmos not only displays a superb role with uncomparable quality in cimparison to any conventional diagnostic mehtods but also has a decisive role in determining the therapeutic aims and the appropriate operative method when the operation is indicated. RESULT : 1. The etologic disease of exophthalmos were 6 cases of pseudotumor, 4 cases of thyro양 ophthalmopathy, 4 cases of maxill ary sinus and nasal cavity Ca., 3 cases of mucocele and 1 case of alveolar soft part sarcoma, osteoma. dermoid cyst, pleomorphic adenoma, meningioma, and C.C.F.each. 2. The origin of he etiologic diseases of exophthalmos were 13 cases of primary within bony orbit and 10 cases of secondary from adjacent structure. 3. The sites of lesions were 11 cases of intraconal andextraconal, 10 cases of extraconal, and 2 cases of intrconal origin. 4. The degree of exophthalmos on CT scan was in proportion to the volume of the mass except in the case of thyroid ophthalmopathy. The upper limit of mormal range by CT scan using regression line equation was 16.2 mm in approximation. 5 CT was a very useful diagnostic tool in the accurate assessment of the kinds of lesion, its location, and its relationship to adjacent structures in the diagnosis of etiologic diseases of exophthalmos.