Breast cancer is the most common cancer form in women. Numerous biological factors have now been identified and accepted as important risk factors and prognosticators. Psychosocial factors are also considered to be of probable importance. A review of the literature studying these factors reveals major methodological problems in evaluating data: small sample size, retrospective design, lack of cross-referencing for other important factors, cross-sectional studies instead of longitudinal studies, and insufficient statistical analysis. Regarding psychosocial factors, some of the most valid studies indicate that the risk of getting breast cancer may be connected with difficulties in expressing feelings, especially ones of aggression; while coping strategy, amount of stress, and level of activity, seem to be of possible influence to the prognosis. A possible connection between psyche and the immunological system has been proposed, but there has been little data so far. Although a series of studies have shown some influence of psychosocial factors on breast cancer, the methodological problems are so large as to leave open the question whether psychosocial factors have any impact upon the disease.