The most important cues to the normal vergence response are retinal disparity, accommodative effort and perceived distance (proximal cue). In the past, proximal cues have been thought to contribute little to the total vergence response but more recent studies have suggested that they play an important role. The relative contributions of the proximal, accommodative and disparity cues to the vergence response were assessed by measuring the effect upon the vergence system when each of these cues were altered. The effect upon the vergence system was measured by the change in 'phoria, the change of fixation disparity and the amount of prism adaptation after 210 s of binocular viewing. The results indicate that the greatest effect of cue disharmony occurred when the proximal and disparity cues were altered. A finding which signifies the importance of the proximal cue to the overall vergence response.