Flow cytometry is a useful technique for detecting surface antigens on cells in suspension, particularly lymphoid cells in blood or disaggregated lymphoid tissue. It is eminently suited to the diagnosis of lymphoma, but its use in differentiating brain tumors depends on better definition of tumor cell surface antigens. Flow cytometry's best application is in measuring cellular DNA content and proliferative activity of tumor cells, and, therefore, it can readily detect whether a tumor is diploid or aneuploid. Clinical correlations of aneuploidy in astrocytomas are still controversial, but there is significant evidence relating aneuploidy to a favorable prognosis in patients with neuroblastoma and medulloblastoma. Studies in which the proliferation fraction was assessed by means of BUdR incorporation or using the monoclonal antibody Ki-67 indicate a marked correlation with biological behavior in a variety of brain tumors.