Summary—Flow cytometry (FCM) analysis of cellular DNA content has become an increasingly important clinical research tool for the measurement and identification of abnormal cell populations. Examination of cellular DNA content can provide information for use in cell cycle analysis in cancer studies. Most researchers would agree that tumor cell proliferation indices should be leading candidates in the search for prognostic factors in breast cancer. FCM is certainly a powerful technique but the assignment of DNA index and synthetic phase of cycle are subject to a number of technical pitfalls which limit the accuracy and reproducibility of this information. This paper will focus on the difficulties encountered in FCM‐DNA analyses performed in a clinical cont