Cytogenetic investigations of neoplastic cells during the past 25 years have revealed more than 600 acquired, recurrent, balanced chromosome rearrangements, and it has been established that every tumor type, studied in a sufficient number to permit conclusions, may be subdivided on the basis of specific, and even pathognomonic, abnormalities. At the molecular level, the balanced rearrangements exert their action through one of two alternative mechanisms: Deregulation of one gene by relocation to an immunoglobulin or T-cell receptor gene, or the creation of a hybrid gene by the fusion of parts of two genes. At present, nearly 100 genes have been found to be involved in neoplasia-associated chromosomal rearrangements, the great majority in hematological disorders. At the same time, the clinical usefulness of various cytogenetic abnormalities as diagnostic and prognostic aids has been increasingly appreciated. The identification of a recurring chromosome abnormality can assist in the diagnosis and subclassification of a malignant disease and, hence, in the selection of the appropriate treatment. The karyotype is also an independent prognostic factor. In hematological neoplasms, where the knowledge of chromosome abnormalities still is much more complete than is the case with solid tumors, cytogenetic analysis now plays an integral part in the diagnostic work-up of individual patients. Data obtained during recent years strongly suggest that corresponding breakthroughs will be achieved in solid tumors within a not-too-distant future.