To investigate the incidence rate of inferior oblique muscle overaction (IOOA) in the contralateral eye and the effect of inferior oblique (IO) muscle recession of the contralateral eye in the patients who received IO muscle recession for unilateral moderate (+2 or +3) primary IOOA.

Medical records of 88 patients with unilateral primary IOOA who underwent unilateral IO muscle recession were retrospectively reviewed and observed during a follow-up period of more than 1 year. Graded recession of the IO muscle was performed according to the degree of IOOA. If postoperative IOOA was below +1, the surgery was considered successful. If IOOA in the contralateral eye was more than +2 after surgery, the IOOA was considered to have occurred.

In cases where IOOA was +2 and +3 before the surgery, the success rate was 98.2% and 100%, respectively, showing an overall success rate of 98.8%. The incidence of contralateral IOOA after recession of the unilateral IO muscle was 24.1% in the +2 group and 16.6% in the +3 group with an overall incidence rate of 21.6% and when the IO muscle recession of the contralateral eye was performed, the success rate was 100%.

The incidence of contralateral IOOA was 21.6% after the unilateral IO muscle recession in moderate unilateral primary IOOA. A satisfactory outcome was obtained through an additional IO muscle recession in the contralateral eye.