Thkek chromosomes of about 40 human malignant gliomas in adults have been studied. Three tumours had double-minutes (dms) in all or most of their cells whereas 5 additional gliomas showed the same aberration in sporadic cells. The humber of dms varied between 1 and more than a hundred. In most case the majority of the double-minutes were extremely smasll and they were morphologically similar to those seen in some murine sarcomas. Only a few of the dms seemed to be centric. The occurrence of dms could not be related to any particular deviations as regards ordinary chromosomes. Some observations suggested that the dms could be derived drom one or a few pulverized chromosomes. The period the dms could remain in the tumour cell population is probably limited. This problem is discussed together with a possible mechanism for their initial increase in number in the tumour cells.