This study was designed to investigate the effect of induced vertical disparity by prisms on horizontal fusional reserves at near distance. One hundred and seventy students were recruited and vertical disparity was induced by small vertical prisms of 0.5 and 1PD base-down in trial frames. Mean age of students was 21±1.24 years and 71.2% were female. Mean near angle of deviation was 4.6±2.4PD exophoria; 88.8% were exophoric and 19 were orthophoric. Mean blur, break and recovery points of positive fusional vergences at baseline were 11.1, 24.2 and 18.9PD; with 0.5PD vertical prism, 11, 23.3 and 18.5PD; and with 1PD vertical prisms, 10.5, 23 and 18.2PD. Mean blur, break and recovery points of negative fusional vergences at baseline were 9.5, 17.1 and 12.9PD; with 0.5PD vertical prism, 8.5, 16 and 12.2PD; and with 1PD vertical prisms, 8, 15.1 and 11.2PD. The authors conclude that induced vertical disparity reduces horizontal prism fusion reserves at near fixation with the greatest effect on negative ranges. Small vertical deviations should be considered when treating symptomatic horizontal deviations.