The nuclear DNA content (DNA ploidy) level was determined in a series of 83 human brain metastases, for which 35 complete clinical follow-ups were available. The DNA ploidy level determination was carried out by means of DNA histogram types. The results show that certain brain metastases were diploid, while others exhibited aneuploidy levels ranging from low to very high. The present study also shows that a significant proportion, i.e. 18%, of the 83 brain metastases, exhibited very high levels of aneuploidy, i.e. hypertetraploidy, hyperpentaploidy and octoploidy. We have previously observed that this feature appeared only rarely, i.e. in less than 2% of primary nervous tumours. Furthermore, the present study shows that DNA ploidy level in brain metastases is related significantly (P < 0.001) to patient survival. Indeed, while 9/13 (69%) patients with diploid brain metastases survived longer than 9 months, none (0%) of the 22 patients with aneuploid brain metastases survived longer than the 9 months following the diagnosis of their brain metastases.