Chromosome studies in acute leukaemia, prior to the introduction of banding techniques, showed that approximately 50% of patients had chromosome abnormalities in their leukaemic cells. In general these abnormalities were marked hyperdiploidy in acute lymphoblastic leukaemia, and hypodiploidy or mild hyperdiploidy in acute non-lymphoblastic leukaemia. In some cases, both normal and abnormal cell lines existed together in the one patient. No cytogenetic change was found in the other 50% of patients. With the increased precision of chromosome identification made possible by banding techniques, many of the varied changes previously described in acute leukaemic cells are found to be non-random. The chromosomes involved specifically are 5, 7, 8, 9, and 21, with alterations being either in number, mainly trisomy or monosomy, or in structure, i.e. deletion or translocation. Additional random changes also occured. It is now possible to assign specific karyotypes to certain types of leukaemia, thus making the cytogenetic analysis of bone marrow cells in acute leukaemia a useful diagnostic procedure. A lack of mitoses in chomosome preparations at diagnosis appears to carry a poor prognosis, as does the presence of many random chromosome changes in the leukaemic cell lines. Chemotherapy may be altered according to the detection of these features. The fact that non-random chromosome changes occur in acute leukaemia and in the acute or blastic phase of chronic granulocytic leukaemia, suggests that a genetic factor may be involved in the development of the disease. In addition there appears to be a relationship between virus combining sites and virus induced changes in particular human chromosomes, and the non-random changes present in leukaemia.