A study was made of the inter-relationships and prognostic significance of structural characteristics found in primary breast cancers and their associated axillary lymph nodes. The prognostically favorable characteristics included the following. For the primary tumor: nuclear differentiation of the cancer cells, diffuse lymphoid cell infiltrations (LI), and perivenous lymphoid cell infiltrations (PVI). For the axillary lymph nodes: sinus histiocytosis (SH). Perivenous lymphoid cell infiltrations (PVI) in the primary tumor are found to be as important a prognostic factor as SH in the axillary lymph nodes; these two characteristics are found to be positively associated. Evaluation of the nuclear grade (NG), LI, and PVI in the primary tumor allows for the definition of prognostically significant patient groupings. LI is found to occur primarily in assoication with cancer cells having a low (anaplastic) nuclear grade, and to be positively associated with follicular hyperplasia (FH) in the lymph nodes. We also found a positive association between the cellular responses to areas of in situ carcinoma and the cellular responses to accompanying invasive breast cancer. These findings add to our general understanding of immunogenicity of breast cancer tissue. They also provide a progonstic system for classifying breast cancer patients on the basis of the microscopic characteristics of the primary tumor and surrounding breast tissue. The latter system should be of value in comparing the therapeutic benefits of various treatments.