From an initial group of 39, 16 cycling peri-menopausal women completed a longitudinal study in which they recorded menstrual and sexual behavior daily and were interviewed at roughly 4-mth intervals until 1 yr or more without cycling. At each interview women gave 20-ml blood samples, completed sexuality questionnaires, and rated themselves for menopausal symptoms. As predicted, the difference in weekly rate of sexual intercourse before and after the cycle showed a significant decline (P less than 0.05). For each subject, mean weekly rates of sexual intercourse for 13-wk periods over the entire transition period were plotted and the slope of the line was calculated. Overall, the mean slope was negative, as predicted, and was significantly different from zero (P less than 0.05). The questionnaire data showed that compared with their pre-menopause data, the women had fewer sexual thoughts or fantasies (P less than 0.01), suffered more from lack of vaginal lubrication during sex (P less than 0.01), and were less satisfied with their partners as lovers (P less than 0.05) after menopause. While estradiol (E) and testosterone (T) levels showed significant declines (P less than 0.02), testosterone showed the most consistent association with coital frequency. The findings generally supported our initial hypothesis of a decline in sexual interest and coital frequency after menopause.