The chromosome abnormalities associated with the classical type of chronic granulocytic leukaemia (CGL) involve a deletion of chromosome number 22 (Ph1) and translocation of the deleted material to another autosome, usually a number 9, t(9;22) (q34;q11). There is no clinically detectable difference between patients showing usual or unusual sites of translocation of 22q. In the chronic phase the typical translocation without additional abnormalities accounts for about two-thirds of the cases. Additional abnormalities include -Y and +8; difference of prognosis in association with aneuploidy has not yet been demonstrated. The presence of Ph1-negative cells in the chronic phase may be related to the time of ascertainment. The isochromosome for the long arm of number 17 is pathognomonic of impending metamorphosis. Additional chromosomal abnormalities are found in the majority of cases in the acute phase. Recent advances in immunological techniques for distinguishing different cytological types of blast cells, in patients who present with acute leukaemia with the Ph1 chromosome, have provided insight into the relationship of the Ph1 chromosome to the pathogenesis of the disease.