Abstract Chromosome studies in hematological disorders made rapid and significant advances following the introduction of banding techniques and the most salient features of these studies are succinctly reported in this Review. Considering the consistency of the chromosome anomalies first, it appears that about 50% of the acute leukemias do not present with or develop detectable karyotypic changes, which could lead to the assumption that they may play a role at any level in the development of the malignant process. More recent work, however, seems to indicate that, at least in some cases, the normal metaphases found in the marrow by direct methods and on unstimulated blood may not have originated from leukemic cells. If clonal chromosome anomalies are to be of some significance, they ought to be nonrandom in character and that is what studies of the last few years have shown. Thus, correlations between these chromosomal anomalies and the types of malignant proliferation have been established, i.e., the Philadelphia (Ph 1 ) chromosome and chronic granulocytic (myelocytic) leukemia (CML) plus some cases of acute myeloid or lymphoid leukemia; translocation t(15;17) and acute promyelocytic leukemia (APL), including the typical form as well as its morphological variants; translocation t(8;21) with or without loss of a sex chromosome and acute myeloblastic leukemia (AML) with some maturation; structural anomalies of the long arm of chromosome #11 and acute undifferentiated monoblastic leukemia; loss or deletion of chromosomes #5 and #7 in acute nonlymphocytic leukemia (ANLL); translocation t(8;14) and its variants, but always including chromosome #8, in Burkitt lymphoma and Burkitt-type acute leukemia; and chromosome 14q+ in lymphoid proliferative disorders, especially of the B-cell type. Other examples of nonrandom anomalies are found in acute lymphoblastic leukemia (ALL) (deletion of the long arm of chromosome #6; translocation t(4;11)), in chronic lymphocytic leukemia, certain myeloproliferative syndromes, and in preleukemia (the 5q− anomaly). The relationship between constitutional and acquired chromosomal anomalies (trisomy and acute leukemia), as well as between chromosome breakage syndromes and malignant proliferation, is discussed. Modification of the chromosome changes during the course of the disease have also contributed greatly to our better understanding of these conditions: disappearance of the anomalies during remission and reappearance in relapse of acute leukemia and clonal evolution, allowing critical evaluation of the relationship between the presence of consistent nonrandom chromosome anomalies and prognosis of the disease. Finally, the practical indications for chromosome investigations in hematological disease are listed, and certain future developments are mentioned: study of the relationship between chromosome anomalies and possible etiological agents, biochemical and functional consequences of chromosomal anomalies, improvement of present techniques and the application of new ones and reevaluation of chromosome methodologies in relation to the kinetics of the malignant cell proliferation.