Investigators are currently using techniques of DNA flow cytometry to measure ploidy status (DNA content) and proliferative potential (S phase fraction) in a wide variety of solid tumors. These measurements have shown relevance for diagnosis, prognosis, and treatment for patients with cancer. National cooperative group studies are beginning to evaluate these measurements in controlled clinical trials to further define their clinical utility as well as limitations. The purpose of this report is to discuss both practical aspects of DNA flow cytometry and clinical applications of these measurements in a variety of solid tumors. We describe in detail our methods for sample handling, processing, and interpretation of DNA histograms. Although there are multiple ways of processing samples and interpreting histograms, we present methodology and interpretations that have proven efficient and reproducible. The review of clinical applications of flow cytometry to solid tumors focuses on breast cancer, but includes a discussion of other solid tumors.