The successful prescribing of vertical prism for the individual patient requires that (1) the deviation be measured in the presence of horizonal fusion, (2) the deviation be determined with distance and near fixation in the straight ahead and down-gaze positions, and (3) the ability to adapt to the prism be assessed. This study addresses the first requirement by evaluating the measurement of fixation disparity of asymptomatic patients using a readily available clinical procedure. The results indicate that the vertical fixation disparity varies directly and in a consistent manner with forced vertical vergence in patients with no vertical fixation disparity as well as in those with a vertical fixation disparity with zero forced vertical vergence. The implication of this information for the prescription of vertical prism is discussed. © 1983 American Academy of Optometry.