Although findings from recent animal studies suggest that behavioral factors such as "helplessness" play a role in cancer progression, very few such studies with humans have been carried out. The study investigated the predictive power of an immunologic effector cell, the natural killer (NK) cell, as well as selected psychological and demographic factors, to breast cancer prognostic risk status. It was found that NK activity predicted the status of cancer spread to the axillary lymph nodes. Patients who had low levels of NK activity were rated as well-adjusted to their illness; patients who had higher NK activity appeared to be distressed or maladjusted. These findings are discussed in the light of recent animal findings linking environmental stress and behavioral responsiveness to biological vulnerability via endocrine and immune pathways.