Because of the recognized relationship between aging and the development of cancer, the authors have proposed that the rate and extent of involution may be important factors in the etiology of breast cancer. They further have examined existing epidemiologic and histopathologic data to evaluate the possible association between abnormal involution and breast cancer risk. Evidence for the association is largely circumstantial, because studies have not been designed specifically to evaluate the relationship between the process of involution and breast cancer. Until studies are performed to compare histopathologically the extent of involution in breast cancer patients and age-matched controls, the role of involution in the etiology of breast cancer will remain uncertain.