OBJECTIVE: Criteria for staging the menopausal transition are not established. This article evaluates five bleeding criteria for defining early transition and provides empirically based guidance regarding optimal criteria. DESIGN/SETTING: Prospective menstrual calendar data from four population-based cohorts: TREMIN, Melbourne Women's Midlife Health Project (MWMHP), Seattle Midlife Women's Health Study (SMWHS), and Study of Women's Health Across the Nation (SWAN) with annual serum FSH from MWMHP and SWAN. PARTICIPANTS: 735 TREMIN, 279 SMWHS, 216 MWMHP, and 2270 SWAN women aged 35-57 at baseline who maintained menstrual calendars. MAIN OUTCOME MEASURE(S): Age at and time to menopause for: standard deviation >6 and >8 days, persistent difference in consecutive segments >6 days, irregularity, and >or=45 day segment. Serum FSH concentration. RESULT(S): Most women experienced each of the bleeding criteria. Except for a persistent >6 day difference that occurs earlier, the criteria occur at a similar age and at approximately the same age as late transition in a large proportion of women. FSH was associated with all proposed markers. CONCLUSION(S): The early transition may be best described by ovarian activity consistent with the persistent >6 day difference, but further study is needed, as other proposed criterion are consistent with later menstrual changes.