Fundamental principles of flow cytometry with emphasis on DNA measurements and cell cycle analysis in human cells and tissues are summarized. Some of the pitfalls of cell preparation techniques and histogram interpretation are discussed at length. While consensus has been reached for some organs and tumors that DNA quantitation by flow cytometry (or image cytometry) may be of prognostic value, for most cancers studied to date the information remains incomplete. Thoroughly lacking are well-structured prospective studies because retrospective studies, while suggestive, may not necessarily be of the same value. Potential usefulness of other tumor markers is briefly discussed. Many fundamental questions concerning definitions of "diploid" and "aneuploid" tumors have not been satisfactorily settled. While the goal of "objective measurements" is worthy of further pursuit, the interpretation of results is often highly subjective. The biologic reasons for behavioral differences between diploid and aneuploid tumors are still totally obscure.