Seventy-five premenopausal women autopsied under medical examiner auspices were selected for a correlative study of breast and endometrial morphology proceeding through the menstrual cycle. Criteria for selection included adequate preservation of the endometrial and breast tissue, relatively even distribution of women by age (range 15-56), menstrual cycle date, and parity status. Hormonal therapy and disease states that might influence pituitary-ovarian cycling were reasons for exclusion from the study. Proliferative phase breast was characterized by small lobules with few terminal duct structures. Terminal duct epithelial mitoses were uncommon. Intralobular stroma was condensed and continuous with interlobular stroma. Secretory phase breasts were characterized by increasing size of lobules and number of terminal duct structures and duct epithelial basal vacuolization and mitoses. Intralobular stroma became increasingly loose and edematous. Stromal lymphocytic population increased toward the end of secretory phase. Perimenstrual breasts underwent lobular contraction with necrosis and sloughing of duct epithelium. There was a concomitant marked increase in lobular stromal lymphocytic infiltrate and metachromasia. These features heralded a return to the proliferative phase appearance. These marked cyclical changes have implications for routine pathologic diagnosis as well as for the newer noninvasive diagnostic techniques.