This study investigated two hypotheses: (1) sufficient cells may be obtained by needle aspiration of breast nodules to produce good flow cytometric DNA profiles; and (2) benign breast lesions do not produce aneuploid G0G1 peaks, and therefore a distinct aneuploid peak is sufficient for a diagnosis of malignancy. Breast specimens received in Surgical Pathology between December 1985 and February 1987 were aspirated, and the cells stained with propidium iodide for flow cytometric DNA analysis. A total of 344 specimens were aspirated, of which 204 (59%) were malignant and 140 (41%) benign. One hundred fifty-three malignant and 111 benign specimens contained sufficient cells for analysis. Cytologic smears were available for 177 malignant and 123 benign specimens. DNA histograms were considered diagnostic of malignancy if an aneuploid peak was present which contained at least 20% of the cells in the distribution, and had a DNA index greater than or equal to 1.2. Using these criteria, 73 of 153 (48%) carcinomas could be identified. None of the benign lesions satisfied these criteria. One fibroadenoma with atypical hyperplasia produced a distinct peak which contained less than 5% of the cells in the histogram, and had a DNA index of 1.25. Flow cytometric analysis provides objective data that complement the subjective cytologic interpretation of fine needle aspirates.