Although the importance of the relationship between age and cancer incidence rates is recognized universally, little attention seems to have been paid to the relationship between the physiologic aging process in the breast and breast cancer risk. The natural history of breast cancer was examined in the context of involution of the breast, and it was proposed that the rate and extent of involution might be important determinants of breast cancer risk. It was concluded that additional studies to characterize the process of involution better histopathologically and to examine the associations between the rate and extent of involution and breast cancer risk are warranted.