Morphological identification of cell multiplication (mitosis) and cell deletion (apoptosis) within the lobules of the "resting" human breast is used to assess the response of the breast parenchyma to the menstrual cycle. The responses are shown to have a biorhythm in phase with the menstrual cycle, with a 3-day separation of the mitotic and apoptotic peaks. The study fails to demonstrate significant differences in the responses between groups defined according to parity, contraceptive-pill use or presence of fibroadenoma. However, significant differences are found in the apoptotic response according to age and laterality. The results highlight the complexity of modulating influences on breast parenchymal turnover in the "resting" state, and prompt the investigation of other factors as well as steroid hormones and prolactin in the promotion of mitosis. The factors promoting apoptosis in the breast are still not clear.