This paper reviews the chromosomal and cytological patterns in acute leukemias (AL) and attempts to establish a correlation with the FAB classification. In fact, these studies are useful in distinguishing different forms of AL and in understanding the nature of the cell from which the malignant clone originated. As shown by the results obtained from the study of acute promyelocytic leukemia, for instance, these studies also provide a likely explanation for the lack of chromosome abnormalities in certain cases of AL.