The modern techniques for assisted reproduction have exposed many women to potent follicular stimulation regimens. Follicular stimulation is associated with an exposure to potent fertility drugs and abnormal estrogen, progesterone and human chorionic gonadotropin (hCG) concentrations. These treatments and hormones might affect the proliferation of epithelial breast cells and thus the risk of breast cancer. The possible effects of ovarian stimulation on the incidence and course of human breast cancer, and the need for long-term cohort studies is discussed.