Analysis of cellular DNA content by cytometry is important in clinical and biological research. Measurements are used widely to assess the relative DNA content of tumor stemlines and to assist in the detection and evaluation of malignant diseases. A review of the literature on DNA measurements in solid tumor and leukemias reveals a confusing variety of terms applied for the description of similar results. In order to facilitate the understanding of data and to standardize the terminology for DNA analyses, a questionnaire was distributed to more than 500 investigators. Subsequently, a workshop on terminology was held at the Combined Conference on Analytical Cytology and Cytometry IX and VIth International Symposium on Flow Cytometry, Schloss Elmau, West Germany, 18-23 October, 1982. The workshop nominated a nine-member committee to develop guidelines for nomenclature to be used in reporting results from analyses by DNA cytometry. The committee was charged by the Council of the Society for Analytical Cytology to complete this task and to publish its recommendations in Cytometry and in Cancer Genetics and Cytogenetics. The following guidelines are based on the questionnaires returned and the discussion at the workshop; they represent the unanimous recommendations of the committee. The five guidelines given herein apply to measurements of relative DNA content of cells that have been stained appropriately and analyzed by cytometry.