Correlating the CT scan features of patients with orbital Graves' disease with histopathologic observations allows one to focus more specifically on the distinguishing features of this disease with future research implications. Both CT scanning and pathologic studies have shown clearly that the extraocular muscles are the primary focus of the disease. Swelling of the extraocular muscles generally occurs within their bellys with sparing of the tendons. This contrast with idiopathic inflammation of the muscles or myositis, which tends to involve the tendon as well. All of the associated findings in orbital Graves' disease probably flow from the enlarged volume of the extraocular muscles: proptosis, bowing of the medial lamina papyracea to accommodate the swollen belly of the medial rectus muscle, venous engorgement from stasis induced by direct compression of the orbital venous drainage, conjunctival and lid swelling, and lacrimal gland enlargement. Both radiographic and pathologic changes in the orbital fat are secondary and comparatively insignificant. While there appears to be no selective inflammation of the optic nerve meninges or the perineural connective tissues, enlargement of the extraocular muscle bellys where they converge at the crowded orbital apex brings about compression of the optic nerve, impairs its function, and causes visual decrease. Lymphocytic and plasmacytic infiltration along with edema within the endomysium of the extraocular muscles leads to the activation of fibroblasts with the production of acid mucopolysaccharides and progressive fibrosis. It is not known what attracts the lymphocytes to the extraocular muscles, why certain extraocular muscles are affected preferentially, why the disease may be asymmetrically unilateral, and whether a defect in T cell or B cell functions (or both) is immunologically at fault.