Chronic myeloid leukemia is characterised by two discrete phases, a 'benign' phase which terminates into an 'acute' phase. Various explanations have been given to explain the cause of 'blastic' crisis in CML. But the consistency and regularity with which blast crisis occurs and the irregularity with which the factors which are ascribed to cause it (e.g. additional chromosomal abnormalities, change in bcr/abl rearrangement, etc. occur, suggests that CML-BC is not a stochastic process in the natural history of CML but is predetermined at the time of the first mutation in the stem cell. A hypothetical model is put forward proposing this. Different points supporting the model are discussed. The most important implication of this model would be to provide an insight that should lead to the development of more selective and appropriate treatment strategies for this disease.