This longitudinal prospective study was designed to examine the influence of menstrual cycle phase on female sexuality. Sixty-nine normally cycling women monitored their sexual behavior including both heterosexual and autosexual activities and their basal body temperature daily over two or three consecutive menstrual cycles. Temperature charts were used to ascertain five cycle phases (menstrual, postmenstrual, ovulatory, luteal and premenstrual) and data were analyzed using analyses of variance with repeated measures. The results supported a significant cyclic fluctuation in sexual behavior with both female-initiated and male-initiated sexual activities peaking in the ovulatory phase. Moreover, the findings indicated that women experienced increased sexual arousal and sexual pleasure as they progressed from the menses to the premenstruum. These data suggest that human sexual behavior may be influenced by hormonal fluctuations and cognitive factors associated with the menstrual cycle.