PURPOSE To determine whether unilateral strabismus surgery creates lateral incomitance in patients with exotropia. METHODS Patients >7 years of age with intermittent or constant exotropia who underwent unilateral horizontal rectus muscle surgery between December 2009 and January 2012 were prospectively evaluated. Prism and alternate cover testing was performed with distance fixation in primary position, right gaze, and left gaze after 1 hour of monocular occlusion. Measurements were obtained within 1 month prior to surgery, within 1 week after surgery, and >3 months after surgery. The surgical procedure varied according to the surgeon's discretion. The change in deviation induced by strabismus surgery in lateral gaze was expressed as a percentage of the change in deviation induced in primary position. RESULTS A total of 12 patients met inclusion criteria. Of the 11 patients with postoperative examinations within 1 week after surgery, 10 (91%) had greater surgical effect with gaze toward the operated eye (P = 0.007). All 9 patients with >3 months' follow-up had greater surgical effect with gaze toward the operated eye (P = 0.003). On average, the surgical effect in gaze toward the operated eye was 120% of that achieved in primary position; in gaze away from the operated eye, 75% (P < 0.001). Three patients had diplopia in lateral gaze toward the operated eye that remained >6 months after surgery. CONCLUSIONS Unilateral strabismus surgery induces lateral incomitance that may cause diplopia >6 months after surgery in patients with exotropia. This should be considered when planning strabismus surgery and counseling patients.