Leptin, an adipocyte hormone, is a trophic factor for the reproductive system; however, it is still unknown whether there is a dynamic relation between fluctuations in circulating leptin and hypothalamic—pituitary–ovarian (HPO) axis hormones. To test the hypothesis that fluctuations in plasma leptin concentrations are related to the levels of luteinizing hormone (LH) and estradiol, we sampled plasma from six healthy women every 7 min for 24 h during days 8–11 of the menstrual cycle. Cross-correlation analysis throughout the 24-h cycle revealed a relation between release patterns of leptin and LH, with a lag of 42–84 min but no significant cross-correlation between LH and estradiol. The ultradian fluctuations in leptin levels showed pattern synchrony with those of both LH and estradiol as determined by cross-approximate entropy (cross-ApEn). At night, as leptin levels rose to their peak, the pulsatility profiles of LH changed significantly and became synchronous with those of leptin. LH pulses were fewer, of longer duration, higher amplitude, and larger area than during the day. Moreover, the synchronicity of LH and leptin occurred late at night, at which time estradiol and leptin also exhibited significantly stronger pattern coupling than during the day. We propose that leptin may regulate the minute-to-minute oscillations in the levels of LH and estradiol, and that the nocturnal rise in leptin may determine the change in nocturnal LH profile in the mid-to-late follicular phase that precedes ovulation. This may explain the disruption of hypothalamic—pituitary–ovarian function that is characteristic of states of low leptin release, such as anorexia nervosa and cachexia.