This paper presents analyses from perhaps the largest and most comprehensive prospective cohort study of mid-aged women--the Massachusetts Women's Health Study (MWHS)--with numbers sufficient to provide, for the first time, stable estimates of parameters in the normal menopause transition. The three questions addressed in this analysis are (i) what are the natural menopause transitions and when do they occur, (ii) what factors affect these transitions and (iii) what signs and/or symptoms accompany these transitions? The data were obtained primarily from 5 years of follow-up of 2570 women in Massachusetts who were aged 45-55 years as of January 1, 1982. An initial baseline cross-sectional survey (T0) yielded a total of 8050 completed responses with an overall response rate of 77%. From this cross-sectional sample a cohort of approximately 2570 women was identified, consisting of women who had menstruated in the preceding 3 months and who had not undergone removal of the uterus and/or ovaries. Prospective study of the cohort consisted of six telephone contacts (T1-T6) at 9-month intervals with excellent retention of the respondents. A subset of the full cohort was defined that consisted of women who were premenopausal (rather than perimenopausal) at baseline (T0) (n = 1178). Confirming prior reports, the age at natural menopause occurred at 51.3 years with a highly significant median difference (1.8 years) between current smokers and non-smokers. The new analyses reported here on median age at inception of perimenopause (47.5 years) and factors affecting it are consistent with findings for age at last menstrual period, particularly the overwhelming effect of smoking. Smokers tend to have not only an earlier but also a shorter perimenopause. The length of the perimenopausal transition (estimated at nearly 4 years) has not been previously reported. Moreover, the highest rate of physician consultations is observed among those with longer perimenopause transitions. The relationship between menopause transitions and symptom reporting appears to be transitory, with reported rates showing an increase in the perimenopause and a compensatory decrease in the postmenopause. The implications of combined hormone replacement therapy for future research on menopause in industrial societies is discussed in relation to these findings.