Aneuploidy is a state of abnormal and highly variable DNA and chromosome content found in both hereditary disorders and human malignancy. For two decades flow cytometry has allowed a wide-ranging survey of aneuploidy in clinicopathological series. Although up to 75 per cent of all tumours analysed display aneuploidy, its value as a clinical marker of biological aggressiveness is still uncertain. New technologies promise to reveal more precisely the genetic and subchromosomal changes that constitute aneuploidy and contribute to the malignant phenotype in human tumours.