This report concerns the distribution of the DNA content in cells obtained from seven nonneoplastic, human brain tissue specimens and from eight different kinds of malignant human brain tumors of 30 patients. Analysis was carried out by flow cytometry suing suspensions of single separated cells stained with propidium iodide as DNA-intercalating fluorochrome. Normal, non-stimulated lymphocytes served as diploid controls. An average of 91% of the cells from non-neoplastic brain tissue was diploid (2C) and these cells were presumable in G0 or G1 stage of the cell cycle. In specimens from malignant brain tumors the proportion of diploid cells in G0 or G1 stage varied between 35 and 89%. In these specimens polyploid (including triploid, tetraploid and hypertetraploid) cells were frequently observed. From the distribution of their DNA content the 14 specimens of glioblastomas were classified into 3 types. Specific patterns were noted in type III-b which was characterized by two peaks having the same heights. The cerebral metastasis displayed the greatest variations in ploidy and these tumors, on basis of their DNA distribution were divided into 3 types. As shown in the present study the cytofluorometric analysis of malignant brain tumors revealed characteristic features: 1) multiple DNA peaks, 2) wide ploid variations ranging from 2C to 8C, 3) variability of DNA content patterns and distribution, 4) relatively large portions of cells in S and G2 + M phase of the cell cycle, 5) presence of heteroploid and polyploid cells. Because flow cytometry has distinct advantages over other methods in permitting rapid analysis of large populations of cells with a high degree of reproducibility, results obtained with this technique may be of significant clinical importance.