In gynecologic oncology valid prognostic factors are necessary to estimate the course of disease and to define biologically similar subgroups for analysis of therapeutic efficacy. The presented study is a prospective study concerning prognostic significance of DNA ploidy and S-phase fraction in breast cancer following enrichment of tumor cells by cytokeratin labelling. Epithelial cells were labeled by FITC-conjugated cytokeratin antibody (CK 5, 6, 8, and CK 17) prior to flow cytometric cell cycle analysis in 327 fresh specimens of primary breast cancer. Univariate analysis in breast cancer detected the prognostic significance of DNA-ploidy, S-phase fraction and CV (coefficient of variation) of G(0)G(1)-peak of tumor cells for clinical outcome, especially for nodal-negative patients. Multivariate analysis could not confirm prognostic evidence of DNA-ploidy and S-phase fraction. In conclusion, in breast cancer no clinical significance for determination of DNA-parameters was found.