The bulk of cancer research has productively focused on the pathophysiology of the disease, emphasizing tumor biology, especially tumor characteristics such as DNA ploidy and estrogen/progesterone receptor status as predictors of disease outcome, at the expense of studying the body's psychophysiological reactions to tumor invasion. These reactions are mediated by brain/body mechanisms, including the endocrine, neuroimmune, and autonomic nervous systems. Although a large portion of the variance in any disease outcome is accounted for by the specific local pathophysiology of that disease, some variability must also be explained by 'host resistance' factors, which include the manner of response to the stress of the illness. The evidence of links between social support, stress, emotional state, and immune and endocrine function is growing. Here we examine evidence that 2 especially promising mechanisms, one immune, one endocrine, may mediate the relationship between stress and social support on the one hand and cancer progression on the other. We chose natural killer (NK) cells and cortisol because they are particularly good examples of mediating mechanisms for which there is solid basic and clinical evidence. NK cells are of great interest because they are involved in tumor surveillance, and because their activity can be measured in vitro. Copyright 2001 by W.B. Saunders Company