To determine if angle of deviation changes significantly after diagnostic monocular occlusion (DMO) in patients with BSV (binocular single vision).We retrospectively analysed data from 136 patients with esotropia and 110 with exotropia, divided in BSV and non-BSV. Near and distance angles were measured over 3 visits and then after 2 hours monocular occlusion. Data were analysed with t-test and linear regression.Esotropes with BSV had significantly larger increases in deviation after DMO than non-BSV patients for near (5 PD vs 3 PD, 95%CI 3-7 PD, paired t-test p=0.0318) and for distance (6 PD vs 3 PD, 95%CI 4-8 PD, p=0.005). There was a significant correlation between the amount of change after occlusion and the initial angle for near (p=0.006) and for distance (p=0.010) in BSV patients only.In exotropes, there was no significant difference after DMO between BSV and non-BSV patients for near (p=0.09) and distance (p=0.532). There was a significant correlation between initial near angle of deviation and change in deviation after DMO in BSV patients(p=0.0009) but not in the non-BSV group (p=0.07). Distance angle of deviation before DMO was significantly correlated to the amount of change in deviation after DMO for both BSV (p=0.022) and non BSV patients (p=0.015).Our study shows that diagnostic monocular occlusion should be performed in all exotropes with initial angles of deviation of less than 30 PD. In esotropes, DMO is more likely to show significant increases in deviation in BSV patients, especially for smaller initial angles (less than 25 PD before DMO). These changes should be taken into account when planning surgery.