AIM: To determine the frequency of extraocular rectus muscle involvement in patients with thyroid-associated orbitopathy (TAO). MATERIALS AND METHODS: A total of 154 orbits of 77 adult patients (53 women and 24 men) with TAO aged from 18 to 81 years (median 49 years) were investigated. Only patients with clear signs of TAO and confirmed thyroid disease who had been referred to the Department of Ophthalmology of the Olomouc University Hospital from May 2007 to December 2012 were included. All patients underwent general ophthalmic examination and ultrasonographic and MRI examinations of the orbit. The largest short and long cross-sectional diameter for every rectus muscle was measured on MRI scans. Spearman correlation analysis was used to determine the correlations between the diameters of rectus muscles and exophthalmos values obtained. RESULTS: A positive moderate correlation (r = 0.514) was shown between the sum of short parameters of all rectus muscles and exophthalmos values. When compared with the normative values and taking gender into account, enlargement of the medial rectus muscle (RM) was found in 55.2 %, of the lateral rectus muscle (RL) in 33.8 %, the inferior rectus muscle (RI) in 57.1 %, and of the superior muscle group (RS) in 59.1 %. In the cases of single-muscle enlargement, the most frequently affected muscle was the RS (48.8 %), followed by the RI (31.7 %) and RM (19.5 %). No case of single-muscle enlargement of the RL was observed. In the cases of two-muscle enlargement, the RS was involved in 64.3 %, the RI and RM in 60.7 %, and the RL in 14.3 %. In the cases of three-muscle enlargement, the most frequently affected muscle was the RM (93.1 %), followed by the RI (86.2 %), RS (69%), and RL (51.7 %). CONCLUSION: Our study found that, in cases with single-muscle enlargement in patients with TAO, the vertical rectus muscles were most likely involved. On the other hand, in cases with multiple-muscle enlargement, the muscle most likely involved was the medial rectus muscle. In addition, the superior muscle group was noted to be affected more frequently than reported in the world literature. Key words: thyroid-associated orbitopathy, extraocular muscles, magnetic resonance imaging.