DNA flow cytometry (FCM) has become a routine method in breast cancer diagnosis for evaluation of ploidy and proliferation kinetics (cell cycle analysis). Image cytometry is less practicable and provides less information than flow cytometry. An optimized technique with a low coefficient of variation is required for optimal results in flow cytometry. The S-phase fraction and the proliferation index (sum of S-phase fraction and G2M fraction) provide prognostic and therapeutically relevant information. A profound knowledge of the technique and its limitations is indispensable for the interpretation of FCM results. It remains to be established whether immunohistological evaluation of cell proliferation has the same prognostic value. Future developments are to be expected from multiparametric analysis and the improvement of mathematical analysis of FCM measurements.