Forty-three perimenopausal women kept daily records of menstrual cycles and sexual activity. Data on hot flashes and plasma estradiol and testosterone levels were obtained at two points during the menopausal transition. The prospective data yielded a significant negative association between hot flash ratings and regularity of sexual intercourse at both time points. A significant negative correlation was found between estradiol (in the early part of the cycle) and hot flashes ratings at the first data point only, and positive correlations were found between hot flashes and ratio of testosterone to estradiol (T/E) at both. Frequency of sexual intercourse and level of plasma estradiol were higher, and T/E and hot flash ratings were lower in "early" perimenopausal women who were still having cycles at least once every 30 days, as compared with "late" perimenopausal women who were cycling less often. It was concluded that a close association exists between increasing irregularity of menstrual cycles, hot flashes, declining estradiol levels, and declining frequency of intercourse during the perimenopause. Causal relationships remain to be established.