We exposed standing participants to optic flow in a moving room. Motion sickness was induced by motion that simulated the amplitude and frequency of standing sway. We identified instabilities in displacements of the center of pressure among participants who became sick; these instabilities occurred before the onset of subjective motion sickness symptoms. Postural differences between Sick and Well participants were observed before exposure to the nauseogenic stimulus. During exposure to the nauseogenic stimulus, sway increased for participants who became sick but also for those who did not. However, at every point during exposure sway was greater for participants who became motion sick. The results reveal that motion sickness is preceded by instabilities in displacements of the center of pressure.