The aim of this study was to obtain a better understanding of the role of hormonal factors in breast cancer risk and to determine whether the effect of reproductive events differs according to age at diagnosis. It analysed the effect of age at menarche, age at first full-term pregnancy, number of full-term pregnancies and number of spontaneous abortions both on the overall risk of breast cancer and on its pre-or postmenopausal onset, using the data on 1718 breast cancer cases, obtained from a large sample of around 100 000 French women participating in the E3N cohort study. The results provide further evidence that the overall risk of breast cancer increases with decreasing age at menarche, increasing age at first pregnancy and low parity. No overall effect of spontaneous abortions was observed. The effect of these reproductive factors differed according to menopausal status. Age at menarche had an effect on premenopausal breast cancer risk, with a decrease in risk with increasing age of 7% per year (P50.05). Compared to those who had their first menstrual periods at 11 or before, women experiencing menarche at 15 or after had an RR of 0.66 (95% CI 0.45 -0.97) in the premenopausal group. Age at first full-term pregnancy had an effect on both pre-and postmenopausal breast cancer risk, with significant tests showing increasing risk per year of increasing age (P=0.001 and P50.05 respectively). A first full-term pregnancy above age 30 conveyed a risk of 1.63 (95% CI 1.12 -2.38) and 1.35 (95% CI 1.02 -1.78) in the pre-and postmenopausal groups respectively. A protective effect of high parity was observed only for postmenopausal breast cancer risk (P for trend test =0.001), with point estimates of 0.79 (95% CI 0.60 -1.04), 0.69 (95% CI 0.54 -0.88), 0.66 (95% CI 0.51 -0.85) and 0.64 (95% CI 0.48 -0.86) associated to a one, two, three and four or more full-term pregnancies. A history of spontaneous abortion had no significant effect on the risk of breast cancer diagnosed before or after menopause. Our results suggest that reproductive events have complex effects on the risk of breast cancer. There is a considerable body of evidence that reproductive factors play a role in the aetiology of breast cancer risk (Kelsey et al, 1993). However, the biological mechanism is not yet clearly understood.Results showing the effect of these factors by age or menopausal status at the time of diagnosis may help to give a better understanding of their role.A pooled analysis of the literature showed that early age at menarche and late age at first full-term pregnancy (FFTP) had a greater effect on the risk of breast cancers diagnosed early (i.e. at an early age or before menopause) than on the risk of those diagnosed late (i.e. after 55 or after menopause). In contrast, it has been shown that the protective effect of multiparity is greater on late breast cancers (Clavel-Chapelon and Gerber, 2001). Epidemiological studies reporting on the relationship between breast cancer risk and spontaneous or induced abortions by age have produced con...