Data from a British national cohort of women born in 1946 were used to investigate the hypothesis that the rate of depletion of oocyte numbers is associated with the age at which a woman reaches the inception of the perimenopause. Menopausal status was obtained using an annual questionnaire, sent to the cohort since the age of 47 years, and risk factor information from contacts with the survey throughout earlier life. Parous women entered the perimenopause later than nulliparous women. Those with the most children had the latest perimenopause, where the estimated hazard ratio for women having four or more children compared with those having none was 0.56 (95% confidence interval 0.39-0.81). Women who had a unilateral oophorectomy reached perimenopause earlier then those who had not (hazard ratio = 1.96, 95% confidence interval 1.15-3.35). There was some evidence that early age at menarche and short menstrual cycles were associated with an earlier perimenopause. No relation was observed with oral contraceptive pill use. Results provided some support for the hypothesis under consideration, while estimates of the median age at inception of the perimenopause due to these factors ranged from 46 to 50 years, which is an important variation with respect to later health.