BACKGROUND : Thyroid-associated orbitopathy (TAO) constitutes an immune-mediated inflammation of the orbital tissues of unclear etiopathogenesis. TAO is most prevalent in hyperthyroid patients with Graves' disease (GD); however, severe cases of orbitopathy associated with Hashimoto's thyroiditis (HT) have rarely been described. METHODS : Herewith we report an unusual case of a middle-aged clinically and biochemically euthyroid woman with a stable HT, who developed a severe unilateral left-sided TAO. Thyrotropin receptor antibodies (TRAb) concentration was negative. Intraocular pressure in the left eye was mildly elevated (24 mmHg), while vision acuity was not compromised. Abnormal positioning of the eyeball suggested the extraocular muscles involvement. Unilaterally, von Graefe's, Stellwag's, Kocher's and Moebius' signs were positive. Conjunctival erythema, redness and edema of the eyelid and an enlarged, swollen lacrimal caruncle were visible. She received 4/7 points in the Clinical Activity Scale (CAS) and class IV in the NO SPECS severity scale for the left eye (I-0, II-a, III-0, IV-b, V-0, VI-0). Magnetic resonance imaging (MRI) revealed thickening of the left medial rectus muscle with an increase in T2 signal intensity and prolonged T2 relaxation indicating an active form of TAO. The patient received therapy with glucocorticosteroids intravenously, followed by intramuscular injections with a cumulative dose of 3.24 g of methylprednisolone during a 9-week period with good tolerance. The applied therapy, combined with adequate L-thyroxine substitution, as well as vitamin D and selenium supplementation, resulted in a complete remission of ophthalmic symptoms. CONCLUSIONS : Unilateral exophthalmos in TRAb-negative patients with HT is not a typical manifestation of the disease, and requires a wider differential diagnosis with MRI of the orbits. Scheme of three iv. pulses of methylprednisolone intravenously and the continuation of treatment with im. injections seems to be an effective and safe method of treatment in this group of patients. What is more, adequate vitamin D supplementation and the maintenance of biochemical euthyroidism may help to achieve an ultimate therapeutic effect. Patients with TAO in the course of HT need a careful and continued interdisciplinary approach both ophthalmological and endocrinological. Further studies are needed to elucidate the etiopathogenesis of TAO in TRAb-negative patients.