3D technology emerged as a new source of entertainment these days but unfortunately visual fatigue, headache and eyestrains have been reported by end users. A more severe effect is the development of Visually Induced Motion Sickness (VIMS). We hypothesize that VIMS is caused by rapid adjustment of the pupil diameter of the human eye. To test this hypothesis we have designed an experiment to observe effects of 3D stereoscopy on pupil diameter. The effect of stereoscopy is produced by generating disparities in the images, which cause the person to see depth in them. 3D induces a vergence accommodation response, producing changes in the pupil. We measured the changes in the pupil diameter caused by changes in the disparity of images. Results show that change in disparity causes a significant change in pupil diameter. This may be the cause of visually induced motion sickness. © 2013 IEEE.