Flow cytometric determination of breast tumor heterogeneity. Flow cytometric analysis was done on the DNA content of nuclei obtained from different sites of small breast tumors. Although specimens for analysis were obtained within a few millimeters of each other, dramatic differences were occasionally observed in the DNA histograms. In a limited study involving 141 consecutive breast specimens submitted for flow cytometry, 52% (74) were found to have at least one DNA aneuploid population. In 18% of DNA aneuploid tumors, one or more specimens from areas grossly identified as tumor had no DNA aneuploid population. Because of the proposed correlation of aneuploidy with a poorer prognosis and possible responsiveness to chemotherapy, multiple sites should be assayed when flow cytometric DNA analysis is done. 