PURPOSE: To describe the prevalence of fat and muscle volume (MV) increase in Graves' orbitopathy (GO) patients, calculated from computed tomography scans, and the associated ophthalmic and endocrine characteristics. DESIGN: Consecutive, observational case series. PARTICIPANTS: Ninety-five consecutive Caucasian GO patients attending the thyroid eye clinic. METHODS: Volumetry using age-specific reference values in untreated GO patients who had been rendered euthyroid. MAIN OUTCOME MEASURES: Subgroups in GO and main characteristics. RESULTS: Four subgroups could be distinguished: Group 1, no fat volume (FV) or MV increase (n = 24); group 2, only FV increase (n = 5); group 3, only MV increase (n = 58); and group 4, both FV and MV increase (n = 8). Patients with an increase of MV were older and had higher thyroid-stimulating hormone-binding inhibitory immunoglobulin TBII, more proptosis, and more impaired ductions than those without MV increase. Patients with an increase of FV differed from those without FV increase only in having more proptosis. The clinical activity score did not differ between the 4 groups. CONCLUSIONS: Of these GO patients, 25% have orbital fat and MVs within an age-specific reference range. An increase of the FV is seen in only 14% of GO patients and characterized by proptosis. Muscle enlargement occurs in 70% of patients and is associated with older age, higher TBII values, more proptosis, and impaired motility. FINANCIAL DISCLOSURE(S): The authors have no proprietary or commercial interest in any of the materials discussed in this article.