Purpose: To determine the associated factors of persistent diplopia after retrobulbar anesthesia. Setting: Strabismus Section, Department of Ophthalmology, Seoul Municipal Boramae Hospital, Seoul, Korea. Methods: A prism and cover test in the diagnostic positions of gaze, force-generation test, and forced-duction test were performed in 11 patients with vertical rectus overaction after retrobulbar anesthesia. A Tensilon® (edrophonium chloride) test, thyroid function test, and orbit computed tomography were performed in 8 patients. Results: None of the 11 patients reported diplopia before receiving local anesthesia. Ten patients showed ipsilateral hypertropia with superior rectus overaction and 1 patient, ipsilateral hypotropia with inferior rectus overaction. One patient had partial fibrosis of the medial half of the superior rectus, presumably caused by a bridle suture. Four (50%) of 8 patients in whom a thyroid function test was performed had abnormal findings; 1 had a history of systemic dysthyroidism. Conclusions: In this study, permanent vertical strabismus after local anesthesia was entirely the result of overactive vertical rectus muscles, mostly the superior rectus muscle. Half the patients who had a thyroid function study had abnormal findings.