We investigated whether postural instability can predict motion sickness and studied relations among instability, motion sickness, and vection. Nine men and 4 women (mean age = 19.85 years) were exposed, while standing, to an optical simulation of body sway. Head motion was recorded using a magnetic tracking system. Postural instabilities were observed prior to the onset of motion sickness. Vection was reported by most participants, including all who became ill. A discriminant analysis revealed that parameters of postural motion accurately predicted motion sickness. The results confirm that postural instability precedes motion sickness and suggest that measures of postural motion may serve as reliable predictors of motion sickness. Potential applications of this research include the development of on-line diagnostic tools that will allow for the prevention of motion sickness in operational and training settings.