An extensive set of prospective data was examined for evidence of covariation between the timing of sexual desire and a number of menstrual cycle characteristics. In any given menstrual cycle, sexual desire was usually first experienced a few days before the basal body temperature (BBT) shift, around the expected ovulation date. Furthermore, positive correlations were found between the day of the BBT shift and the day of sexual desire onset, and between the length of the menstrual cycle and the temporal lag between the onset of sexual desire and the BBT shift. These results are consistent with a model in which sexual desire is affected by the same process that regulates the menstrual cycle. This process is presumably hormonal in nature, suggesting that hormonal factors contribute to sexual desire.