The purpose of this study was to determine if any relation exists between the ability to adapt to vertical prism and the slope of the fixation disparity curve, and if ocular discomfort, while wearing the inappropriate vertical prism, is correlated with the adaptation ability. Patients who showed the greatest adaptation ability generally had flatter slopes than patients who showed the least adaptation ability. The presence or absence of symptoms did not indicate how the patient would adapt to vertical prism. We conclude that greater care and concern should be used in the consideration of an induced vertical prism if the slope of the fixation disparity curve is steep. © 1985 American Academy of Optometry.