The use of flow cytometry for the prognosis of stage II adjuvant treated breast cancer patients. Characterization of breast cancer cells by histology, flow cytometry, and steroid receptors was performed on 197 Stage II breast node positive cancer patients given adjuvant chemotherapy, plus tamoxifen for patients with positive hormone receptors. Histologic and steroid receptor assays were performed using standard techniques; flow cytometric analysis was performed from paraffin-embedded blocks obtained from the primary tumor. Quality control studies on reproducibility, tissue heterogeneity, and analysis procedures have been included. Of the 197 patients studied, aneuploidy was found in 102 (52%); the median %S value was 8% with a range of 0.4% to 38%. Our results demonstrated that number of positive nodes, receptor status, and grade were of prognostic value. Cell cycle kinetic data were not of independent prognostic value in this series. However, ploidy could differentiate in prognosis in the receptor-negative subgroup. Patients with receptor-negative tumors had a significantly better overall survival if the tumor was diploid in nature. Cell kinetics was not significantly prognostic for either receptor subgroup, although patients with higher %S tended to have better relapse-free and overall survival. This is in disagreement with other studies and may demonstrate that treatment has confounded our results and diminished the ability of flow cytometry data to help predict outcome. 