Imprecise measures of ovulation obscure the relationship between women's sexuality and the menstrual cycle, as does studying women with different contraceptive goals in different social contexts. Here we present a novel noninvasive method to precisely pinpoint the preovulatory surge of Luteinizing Hormone (LH), demarcating hormonally distinct cycle phases with greater than 95% reliability. Women were more sexually active on days prior to and including the preovulatory (LH) surge. This pattern was evident only when women initiated sexual activity and not when their partners did, indicating an increase in women's sexual motivation rather than attractiveness. A second study replicated the 6-day increase in sexual activity beginning 3 days before the LH surge, accompanied by stronger sexual desire and more sexual fantasies. We propose the term 'sexual phase' of the cycle, since follicular phase is over inclusive and ovulatory phase is not sufficient. These findings are striking because the women were avoiding pregnancy and were kept blind to the hypotheses, preventing expectation bias. The sexual phase was more robust in women with regular sexual partners, although the increase in sexual desire was just as great in nonpartnered women, who also reported feeling less lonely at this time. We use these results to evaluate potential neuroendocrine mechanisms underlying women's sexual motivation and activity.