Purpose:To study the histopathologic features of the Mueller muscle in chronic eyelid retraction caused by thyroid-associated orbitopathy. To investigate if the degree of eyelid retraction correlates with any histopathologic finding.Methods:A prospective case series of 23 consecutive patients with thyroid-associated orbitopathy was studied. Specimens were obtained during a standard muellerectomy. Formalin-preserved specimens were studied with the use of hematoxylin-eosin, periodic acid–Schiff, Masson trichrome, and Giemsa stains. Immunostaining against leukocyte common antigen, L26, CD3, and KP-1 was performed. Three control specimens were also evaluated in a similar fashion. Fresh tissue was placed in cold glutaraldehyde overnight, postfixed, dehydrated, and infiltrated with epoxy resin. Silver (70 nm) sections were cut and stained with uranyl acetate and lead citrate for electron microscopic examination.Results:On light microscopy, fibrosis and mast cell infiltration was present in all 23 specimens. Fat infiltration was noted in 16 of 23 specimens and did not correlate with increasing age of the patient. Interstitial edema and lymphocytic infiltration were not observed. On immunohistochemistry, leukocyte common antigen was positive, confirming the presence of inflammation. L26, CD3, and KP1 were negative. Electron microscopy demonstrated fibrosis, mast cells, and abundant contracting Mueller cells. The degree of clinical retraction in millimeters did not correlate with fibrosis, inflammation, or fat infiltration. The control specimens demonstrated rare fat and mast cell infiltration and no fibrosis.Conclusions:Contrary to previous reports, the Mueller muscle is involved in the inflammation and fibrosis that characterizes thyroid-associated orbitopathy. The Mueller muscle is grossly enlarged. On histopathologic inspection, fibrosis, fatty infiltration, and increased mast cell presence accompany focal atrophy of the Mueller muscle. In concordance with prior descriptions, many Mueller cells are in an actively contracting state on electron microscopy.