A-scan ultrasonographic measurements of the differences between nerve and dural diameters of the retrobulbar optic nerve were made in 55 eyes of 30 patients with endocrine orbitopathy. This measurement was found to be enlarged in all cases. Extraocular muscle thickness measurements were also made ultrasonically in these eyes, and the degree of intervaginal distension was found to increase with increasing extraocular muscle involvement. These findings were consistent whether the patients were hyperthyroid, hypothyroid, or euthyroid clinically, and the findings occurred in the absence of any detectable functional optic nerve compromise. The cause of this subdural perineural enlargement remains unknown.