Fifty-eight effusions (42 pleural and 16 ascitic fluids) from patients with and without cancer were analyzed by conventional cytology and the results compared with DNA patterns generated by flow cytometry of 10(4) nuclei and several modes of Feulgen cytophotometry. In 31 patients (24 without evidence of cancer and seven with history of cancer and cytologically negative fluids), the fluids were diploid by flow cytometry. One fluid with atypical cells from a lymphoma suspect was also diploid. Flow cytometry of 26 cytologically cancerous fluids disclosed aneuploid DNA patterns in 16 and diploid patterns in ten. Feulgen cytophotometry of 11 of these fluids (three aneuploid, eight diploid) was performed on nuclear preparations identical to those used in flow cytometry and on restrained smears used for visual analysis. The analysis was performed in two modes: as a study of 500 sequential nuclei in an automated system, mimicking flow cytometry, and visually selected large, presumably malignant nuclei. In nine of the 11 cases, the DNA content of visually selected cancer cells was aneuploid, even though this DNA pattern was not evident in the analysis of 500 sequential cells. In two cases, both diploid by flow cytometry, the Feulgen analysis confirmed the presence of cancer cells in the diploid range. In samples of 10(4) nuclei representing a mixed population of cells occurring in effusions, the presence of aneuploid cancer cells may not be disclosed by conventional flow cytometry. A larger sample of cells, a detailed analysis of DNA histograms, and perhaps sorting of select cells in the hypertetraploid range, may prove essential before flow cytometry can be accepted as a diagnostic tool in the laboratory in the assessment of effusions.