To determine if "euthyroid Graves' exophthalmopathy" is a separate autoimmune disorder, we have examined 6 clinically euthyroid male patients (aged 30-50 yrs) who have never had thyrotoxicosis, with recently developed bilateral exophthalmos (classes 2-4 in the A.T.A. classification). We have evaluated T4 and T3 levels, FTI, TSH response to TRH (200 micrograms i.v.), antithyroglobulin and antimicrosomal antibodies (Tgab and Mab) titres and thyroid stimulating antibody (TSab) activity of patients' IgG. In 5 patients we performed also orbital computerized axial tomography (CAT). The T4 and T3 levels ranged 5.6-7.9 micrograms/d1 and 1.2-2.1 ng/ml respectively; FTI ranged 7-9.7 U. 5 patients showed an impaired TSH response to TRH; only one had a normal TSH increase. The Tgab and Mab titre was undetectable in 5 patients; only one showed high autoantibody titre and developed a clear hypothyroidism 8 months later. TSab activity was detectable in the overall group. The CAT revealed a consistent infiltrative involvement of extraocular muscles in 4 patients; the 5th showed evidence of increased density of retroorbital fat, without any muscle involvement. In conclusion, the presence of TSab activity (6/6), the impaired TRH test (5/6) and the infiltrative retroorbital muscle involvement (4/5) seem to suggest an autoimmune thyroid disease even if apparently normal iodothyronine levels were found.