A new and simple method of assessing reliance on vision for postural control was evaluated in 41 normal subjects. Left-right reversal of peripheral vision induced by a head-mounted mirror device caused an initial dramatic instability in approximately half the subjects, when standing on foam to reduce the value of proprioception. Lateral reversal of central vision by means of a prism device evoked similar responses. Sensitivity to vision reversal was significantly correlated with motion sickness susceptibility. Despite some rapid habituation (partially retained over several weeks) sway remained as great as with eye closure in the anterior-posterior as well as lateral direction, indicating complete suppression of the visual input. Balancing with vision reversal caused a selective decrement in performance of a visuo-spatial memory task, suggesting that coping with misleading visual input may place continuous demands on cortical spatial processing.