BACKGROUND: Binocular summation (BiS), or improvement in binocular vision exceeding the better eye alone, is affected by strabismus. Being easily measured, BiS may be a useful indicator for subjective outcomes like stereopsis in strabismus. This study aims to investigate the relationship between BiS and measures of control of intermittent exotropia (IXT). METHODS: Patients with IXT were recruited before undergoing strabismus surgery and underwent tests of binocular and monocular high- and low-contrast visual acuity, stereopsis at distance and near, and Newcastle Control Score (NCS), a score developed by incorporating home control and clinic control criteria into a control rating scale. BiS was calculated using high-contrast Early Treatment of Diabetic Retinopathy Study (ETDRS) and Sloan low-contrast acuity charts (LCA) at 2.5% and 1.25% contrast as the difference between the binocular score and that of the better eye. The relationship between BiS and measures of IXT control (NCS and distance near stereoacuity disparity) was evaluated using a correlation analysis by Spearman correlation coefficients and the Kruskal-Wallis test. RESULTS: Thirty-four patients were included (mean [± standard deviation (SD)] age 19±16 years) having a mean (±SD) of 26±16Δ IXT at distance and 20±16Δ at near. Mean (±SD) BiS for ETDRS and Sloan LCA at 2.5% and 1.25% was 0.8±3.6, 1.9±6.0, and -2.3±7.2, respectively. The Spearman correlation coefficient of BiS and NCS was -0.53 (95% CI -0.85 to -0.25) for 2.5% LCA and -0.43 (95% CI -0.77 to -0.13) for 1.25% LCA. BiS at 2.5% LCA (P=0.006) and at 1.25% LCA (P=0.029) significantly differed between the groups based on NCS score groupings (1-3, 4-6, and 7-9), with patients who had better control scores having higher levels of BiS. BiS did not differ significantly between patients grouped according to the difference between stereoacuity measured at near versus distance. CONCLUSION: Significantly lower low-contrast BiS in patients with higher NCS may suggest that decreased BiS is associated with less control in IXT. This finding suggests that BiS may reflect control in IXT across a population of patients with IXT.