A short review is given on DNA aberrations and chromosomal composition of malignant human gliomas. By flow cytometric DNA analysis, a wide range of different ploidies has been reported in biopsied gliomas, from diploid to strongly aneuploid nuclear DNA. However, with the preparation and analysis methods used so far, no clear relationship between the type of ploidy and histology or prognosis has been established. A high proportion of glioblastomas is near-diploid, indicating a high degree of biologic malignancy is not necessarily connected to aberration of the nuclear DNA content. It is possible that improved methods giving a higher degree of resolution will allow separation of the near-diploid populations of malignant human gliomas from normal diploid cells and permit the detection of subpopulations with small differences from the dominant DNA mode. Chromosomal studies of malignant gliomas have confirmed that the majority of them have near-diploid stemlines. These populations are seldom normal diploid, however, as both numerical and structural abnormalities are usually present. In addition, chromosomal analyses have shown that when gliomas are bimodal, the polyploid populations are usually doubled versions of the near-diploid ones. In contrast to the near-diploid populations that characterize biopsied malignant gliomas, both FCM studies and karyotyping have demonstrated that permanent cultured cell lines derived from malignant gliomas are usually near-triploid or near-tetraploid. Sequential karyotypic studies of these tumors from biopsy through establishment in vitro have shown an evolutionary pattern consisting of doubling of the original stemline, followed by gains or losses of individual chromosomes with new marker formation in late culture. Evaluation of biopsied malignant gliomas by karyotyping has also demonstrated that subgroups of them are characterized by specific numerical and structural deviations. These groupings may prove useful in predicting prognosis or responsiveness to specific therapeutic regimens. The specific chromosomal abnormalities observed in malignant humam glioma may provide clues as to the genes important in glial transformation. As chromosomal loci for production of structural proteins and enzymes involved in glial metabolism are mapped and patterns of oncogene activation and amplification are determined for human gliomas, the meaning of the nonrandom chromosomal changes seen in these tumors may become clear.