Head and neck paragangliomas: a clinicopathologic study with DNA flow cytometric analysis. A total of 11 head and neck paragangliomas were the subject of pathologic study, including histologic, immunohistochemical, and DNA flow cytometric analyses. We cannot absolutely predict aggressive clinical behavior using histologic parameters alone, but we can use such parameters to segregate patients into low-risk and high-risk groups. Several trends were observed in the current study. Tumors with higher S-phase fractions, G2/M fractions, or aneuploid cell populations tended to behave "aggressively." The presence of sustentacular cells in the primary tumors cannot be used as an absolute indicator of tumor metastatic potential, as two metastatic paragangliomas in this study contained sustentacular cells in both the primary and metastatic lesions. DNA ploidy status cannot be used as an absolute prognostic parameter as the two metastatic tumors were composed of diploid primary and metastatic lesions. The three tumors with aneuploid cell populations showed "aggressive" histologic and clinical features, but the length of the follow-up period for these cases is too limited to draw any conclusions. Although no absolute criteria can be used at present to gauge aggressiveness, close follow-up of these patients is essential, especially if pathologic findings suggest an "aggressive" course (ie, "malignant" histology, higher S-phase fractions, G2/M fractions, aneuploid cell populations, or decreased sustentacular cell density). 