Virtual simulation sickness (VSS) is a form of visually induced motion sickness that can result from immersion in a virtual environment (VE). As in their susceptibility to the sickness induced by real motion, women have been reported to be more susceptible than men to VSS, yet the reason for this difference is not known. The aim of the current study was to investigate the influence of the menstrual cycle on susceptibility to VSS in 16 naturally cycling women and to compare the responses of this group with control groups consisting of 1) 16 premenopausal women taking a combined monophasic oral contraceptive and 2) 16 men. All female participants were immersed in a nauseogenic VE on days 5, 12, 19, and 26 of their menstrual/pill cycle. These days were chosen because they fall in line with peaks and troughs of ovarian hormone levels. Menstrual cycle phase was confirmed by salivary estradiol and progesterone levels. A 4-week "pseudo-cycle" was assigned to the male participants. Hormone analysis revealed that 9 participants in the experimental group had been tested at the desired phases of their cycle. These participants exhibited a significant increase in susceptibility to VSS on day 12 of their cycle. The hormone analysis also showed that the cycles of the 7 remaining members of the experimental group had not precisely followed the expected pattern, and so these people had been tested on days that did not coincide with peaks and troughs of ovarian hormone levels. No consistent variation in susceptibility was observed over the cycle in these volunteers. In addition, no change in susceptibility was observed over the pill cycle of the oral contraceptive group nor over the pseudo-cycle applied to the male control group. The authors conclude that susceptibility to VSS varies over the menstrual cycle as a consequence of hormonal variation.