BACKGROUND: The use of fertility drugs (FDs) is steadily increasing in Western countries and concern has been raised as to the possible impact of fertility treatments on breast cancer risk. METHODS: We analysed this association in the French E3N study. In this prospective cohort, data on treatment against infertility, duration and time of administration were collected at entry through self-administered questionnaires. Cox regression analysis was used to estimate adjusted relative risks (RRs). RESULTS: Among the 92 555 women from the study population, 6602 women were treated for infertility. During the 10 year follow-up period, 2571 cases of primary invasive breast cancer were diagnosed (183 in treated women). Our study showed no overall significant association between breast cancer risk and treatment for infertility (RR = 0.95, confidence interval 0.82-1.11), after surgery or FDs, and whatever the type, the duration of use and the age at first use of FDs. However, infertility treatment was associated with an increased risk, of borderline significance, of breast cancer among women with a family history of breast cancer. This last result had limited statistical power. CONCLUSIONS: Our study provides evidence that treatment for infertility does not influence breast cancer risk overall. An interaction with a familial history of breast cancer is possible but should be investigated further.