This paper presents analyses from a comprehensive prospective cohort study of mid-aged women [the Massachusetts Women's Health Study (MWHS)], with numbers sufficient to provide stable estimates of parameters in the normal menopause transition. Three questions are addressed: what are the natural menopause transitions and when do they occur; what factors affect the transitions; and what signs and/or symptoms accompany the transitions? The data were obtained primarily from 5 years of follow-up of 2,570 women in Massachusetts who were aged 44–55 years as of January 1, 1982. Prospective study of the cohort consisted of six telephone contacts (T1–T6) at 9 month intervals with excellent retention. A subset of the full cohort was defined that consisted of women who were premenopausal (rather than perimenopausal) at baseline (To) (n = 1,178). 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. Smokers tend to have not only an earlier but also shorter perimenopause. The length of the perimenopausal transition, estimated at about 3.5 years, has not been previously reported. The relationship between menopause transitions and symptom reporting appears to be transitory, with reporting rates showing an increase in the perimenopause and a compensatory decrease in postmenopause. The implications of combined hormone replacement therapy for future research on menopause in industrial societies is discussed in relation to these findings.