To investigate how increasing torsional disparity affects clinical measures of the quality of binocular single vision.Synoptophore targets were adjusted to present varying degrees of torsional disparity (relative to the fusion-free rest position of the eyes) such as might be observed in a patient with excyclotropia. This disparity was increased in 2 degrees increments up to 10 degrees in 21 normal subjects. The fusional divergence and convergence amplitudes and random-dot stereoacuity were measured at each level of torsional disparity.Horizontal fusional vergence was not significantly altered from the baseline measures at 2 degrees and 4 degrees of torsional disparity. However, both the divergence and the convergence amplitudes declined and were significantly different than the baseline measures at 6 degrees . A similar decline in the random-dot stereoacuity measurements was evident. Fine levels of stereopsis were decidedly affected at greater than 4 degrees of torsional disparity, while more gross levels were similarly affected beyond 6 degrees .Although human binocular vision can demonstrate the sensory capacity for cyclofusion beyond 10 degrees , torsional disparity of 6 degrees or more significantly degrades horizontal fusional vergence and stereopsis. Torsion in excess of this degree might therefore be a barrier to asymptomatic and comfortable binocular single vision and may require appropriate attention in the planning of more successful surgery for superior oblique palsy and cyclovertical incomitant strabismus.