PURPOSE:To investigate the role of the location of the equator and orientation of newly attached inferior oblique (IO) muscle in the development of contralateral inferior oblique overaction (IOOA). METHODS:Fourteen patients (14 eyes) with 5 to 12 prism diopters (PD) of hyperdeviation in primary position with unilateral, congenital, superior oblique palsy were included. Seven patients underwent modified IO transposition onto the equator (equator group) and seven patients underwent modified IO 14-mm recession (14-mm group). IOOA in the contralateral eye and the angle of strabismus were assessed at 3 months postoperatively. RESULTS:Mean angles of hyperdeviation in primary gaze and sursoadduction were 0.7 and 2.1 PD in the equator group and 0.6 and 2.4 PD in the 14-mm group, respectively. Six patients (86%) developed anti-elevation syndrome and four patients (57%) showed definite 2+ or higher IOOA in the contralateral eye in the equator group. Three patients (43%) in the 14-mm group also developed contralateral IOOA, although it was 1+ or less. The postoperative difference in contralateral IOOA between groups was statistically significant (P = .04). CONCLUSION:This finding suggests that vertical orientation of the IO muscle is another important contributor in the development of contralateral IOOA in addition to the location of the newly attached IO muscle.