Simultaneous flow cytometric DNA content and proliferation-associated nuclear antigen (p105) quantitation was performed on 23 astrocytic tumors and the results correlated with histologic subtype. Three of nine anaplastic astrocytomas and one of ten glioblastomas had an identifiable aneuploid peak, while all four well differentiated astrocytomas were diploid. Cell cycle analysis of malignant gliomas revealed a higher mean percentage of S and G2M cells compared to well differentiated astrocytomas but there was considerable overlap between histologic subtypes. Nuclear antigen analysis of diploid tumors showed a higher mean p105 fluorescence of S + G2M cells than G0G1 cells from the same case but there were no apparent differences in p105 expression by histologic subtype. Aneuploid tumors showed enhanced expression of p105 relative to diploid cells. The findings suggest that the aggressive course of high grade glial tumors may be related to an abnormal DNA stemline or an increase in proliferative activity.