There is currently controversy about the effect of body size on the prognosis of patients with breast cancer. In order to clarify the prognostic importance of body size, and to determine whether the effect varies across subgroups of patients, a critical appraisal of the published literature was undertaken. Thirteen cohort studies and one case-control study were identified that examined the prognostic effect of body size. Methodologic standards were developed that reflected those features of study design considered most important in studies of prognosis in breast cancer, and were independently applied to each report by each of the authors. When the effects of methodologic differences among the studies were controlled, a modest prognostic effect of body size was identified. This effect appeared to be greatest in postmenopausal women, in those with little or no involvement of axillary nodes, and to be independent of other prognostic factors. Additional investigation is recommended to determine the prognostic effects of body size in postmenopausal women with axillary node negative breast cancer and in women receiving systemic adjuvant therapy, and to determine the pathophysiological basis for these effects. Intervention studies to determine the effects of altering body size may also be indicated.