This perspective on Williams et al. (beginning on p. 850 in this issue of the journal) examines the connections between biological responses activated during psychosocial stress and mammary tumorigenesis. Experiments in mouse models of cancer are identifying aspects of tumor biology that may be regulated by hormones such as glucocorticoids released during psychosocial stress. Our growing understanding of the actions of glucocorticoids on breast tumors could lead to important changes in cancer treatment.