The DNA content of selected areas of 16 glioblastoma multiforme predominantly composed of small anaplastic cells was investigated. These findings were contrasted with those of five cases of anaplastic astrocytomas. The microfluorometric determination of DNA was performed on mechanically isolated single cells stained by the Feulgen method, obtained from selected areas of the neoplasms in paraffin blocks. Twelve of 16 cases of glioblastoma multiforme had a main population that was diploid-near diploid. Although four of the five anaplastic astrocytomas disclosed a diploid-near diploid main population there was a more complex DNA distribution than in the small cells in glioblastomas. These differences were evident using two numerical DNA indices obtained from the analysis of each tumor histogram which defined, respectively, the extent to which the main tumor cell population deviated from euploid values and the probabilities to have multiple stemlines with abnormal DNA content. The results suggest that small anaplastic cells which appear to represent the most aggressive population in glioblastomas have a DNA content diploid-near diploid, and in malignant gliomas the degree of aneuploidy of the main stemline is not related to the biologic behavior of the neoplasm.