The use of flow cytometry to analyze the cellular DNA content of human malignancies has become increasingly commonplace. The relationship between abnormalities in DNA content or proliferative characteristics and prognosis is becoming clear for a variety of malignancies in part through new techniques that permit analysis of archival material. High- and low-risk groups of patients with early breast and bladder carcinomas, non-small-cell lung cancer, and colorectal, ovarian, and cervical carcinoma can be distinguished on the basis of abnormal stemline DNA content. In several hematologic and common pediatric malignancies, the prognostic relevance of DNA content flow cytometry has been similarly established. Though the interpretation of tumor cell cycle analyses is less certain, this characteristic may also be prognostically important. However, generalizations cannot be made when applying flow cytometric DNA analysis to clinical decision making. The prognostic importance of an abnormal DNA histogram for an individual patient must be assessed on the basis of the relevant data base for that particular tumor type. The current extent of this data base for various malignancies is reviewed.