To review the prevalence of preoperative and postoperative intorsion in patients with strabismus and Graves' eye disease (GED), and to correlate the intorsion with coexisting superior rectus (SR) and superior oblique (SO) muscle enlargement as a possible mechanism causing intorsion in these patients. Charts of consecutive patients with GED who underwent strabismus surgery between 1 January 2010 and 1 April 2013 were retrospectively reviewed. Of these, patients with orbital CT or MRI scan were identified for further analysis. Clinical characteristics documented included age, gender, horizontal and vertical deviation, subjective torsional deviation, specific extraocular muscles (EOMs) operated upon, EOM enlargement on CT/MRI scans and width and thickness of SO, SR group and inferior rectus (IR). Charts of 45 patients (14 males and 31 females) were reviewed. Mean age was 56.8±12.5 years. Of these, seven (15.6%) patients demonstrated intorsion, and 38 (84.4%) patients demonstrated extorsion preoperatively. But after strabismus surgery, 15 (39.5%) of the 38 patients with preoperative extorsion demonstrated postoperative intorsion and 23 (60.5%) patients continued to show postoperative extorsion. On analysis of CT/MRI scans in these patients, only an increase in the thickness of SR group and the thickness/width of SO muscle were significantly associated with preoperative and postoperative intorsion; while age, gender, preoperative horizontal or vertical deviation and IR recession were unrelated to preoperative or postoperative intorsion. Postoperative intorsion was also associated with smaller degrees of preoperative extorsion (<3.5°). Preoperative SR and/or SO muscle enlargement appear to be a primary contributing factor relating to preoperative and postoperative intorsion in patients with GED-associated strabismus. Patients with only small amounts of preoperative extorsion (<3.5°) in the presence of tight IRs should be carefully evaluated for possible SR and/or SO involvement by CT or MRI scan to predict those at risk for and plan for prevention/treatment of postoperative intorsion.