In a study of the endocrinology of the perimenopausal years, levels of serum FSH, estradiol (E2), immunoreactive inhibin (INH), testosterone, and sex hormone-binding globulin were measured in a population-based sample of 380 women (mean age, 49.4 yr; range, 45.6-56.9 yr). Subjects were divided into women who reported continuing regular menstrual cycles (27%; group I), a change in menstrual flow without a change in frequency (23%; group II), a change in frequency but no change in flow (9%; group III), changes in both frequency and flow (28%; group IV), and at least 3 months since their last menstrual period (13%; group V). After adjusting for age and body mass index, the geometric mean FSH increased across menstrual groups and, compared with group I, was 53% higher in group IV (P < 0.0005) and 253% higher in group V (P < 0.0001). Age- and body mass index-adjusted geometric means for E2 and INH in group V were 54% and 53% of those in group 1, respectively (P < 0.005, P < 0.0001). Women in group V who did not have a menstrual period in the next year had higher FSH and lower E2 and INH levels than those who subsequently went on to have at least one more menstrual period (P < 0.05). FSH was negatively correlated with E2 (r = -0.30) and INH (r = -0.39), whereas INH was positively correlated with E2 (r = 0.45). We conclude that an increase in serum FSH and decreases in E2 and INH are the major endocrine changes associated cross-sectionally with the menopausal transition.