Interest in translational studies aimed at investigating the role of biologic markers in predicting clinical outcome of breast cancer patients and, in particular, response to specific treatments, has progressively increased. Among biologic variables presently under investigation, apoptosis markers, in particular Bcl-2 and Bax expression, are receiving much attention for their relationship with the cellular response to genotoxic damage in experimental tumors. Retrospective, independent studies were carried out by several research groups on about 5000 patients with breast cancer at different stages and with an adequate follow-up. The outcome of separate analyses as a function of treatment generally demonstrated that Bcl-2 overexpression, which correlates with biologic features of a differentiated phenotype (slow proliferation, high steroid receptor levels, absence of p53 and c-erB-2 expression), is associated with a favorable outcome. Such a finding is mainly evident following surgery as well as endocrine treatment. Conversely, no or weak Bcl-2 expression, alone or in association with bax overexpression, appears indicative of a radiation response, and preliminary emerging evidence supports the involvement of such an association of apoptosis-related markers even as predictors of long-term response to neoadjuvant cytotoxic treatment. Although the findings of an involvement of Bcl-2 and Bax as determinants of treatment response should be confirmed within the context of randomized clinical trials, they indicate a combined consideration of proteins that negatively and positively regulate apoptosis in translational studies on the effect of chemical and physical agents at a cellular level.