Women's sexual preferences can change over the hormonal cycle, as several studies, based on responses to questionnaires, diaries, and ratings of photographs, have indicated increased sexual interests around the time of ovulation. However, fewer studies have measured changes in attention or interest to sexually significant stimuli in terms of physiological responses that are not under voluntary control and measure sexual interest indirectly (i.e., without mention of sexual feelings or activities). In the present study, we indexed changes in sexual interest in terms of changes in the eye pupil's size. Pupillary diameter is known to have a proportional relation to the observer's level of interest and attention to a visual stimulus as well as to physical pleasure. Fourteen women (7 being "pill" users) viewed photos on a computer screen while their pupil diameters were recorded using an infrared eye-tracking device. Three measures were taken for each participant during three time windows that estimated the ovulatory, luteal, and menstrual phase of the cycle. We found an increase in mean pupil diameter for sexually significant stimuli during the fertile phase and this pupillary change was also specific to pictures of the participants' actual sexual partners. Moreover, this effect was only seen for women who did not use oral contraceptives. These findings confirm that women's attention for sexually significant stimuli is higher during their fertile phase of the menstrual cycle, and that changes in sexual interest are implicitly measurable using pupillometry.