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Two studies were performed to investigate dimensional stability of the cornea throughout the female menstrual cycle. In the first study, changes of corneal curvature, topography, and thickness were measured for six women for one complete menstrual cycle. Steepening of horizontal and vertical curvatures occurred at the beginning of the cycle, and flattening occurred after ovulation. Corneal thickening occurred on the second day of the cycle and around the time of ovulation, then thinning and another slight thickening on day 21. In the second study, more detailed information was sought on the timing of corneal thickness changes throughout any cycle. Corneal thickness was studied for two subjects throughout three consecutive cycles. Both subjects showed a slight decrease in thickness toward the end of the menses; thickening occurred at ovulation followed by thinning. Thickening also occurred 4 days after ovulation. Consideration of changes in urine levels of estrogen and pregnanediol suggests that rises in estrogen are accompanied by increases in corneal thickness.