Establishment of the exact chromosome constitution in the leukemic cells of CML can serve as a mean of "staging" of the disease. Such "staging" is of more than theoretical value, for it may be utilized not only in the differentiation of the type of CML present, but may also serve as a useful guide in the therapy of the disease. The most important determination is to ascertain whether the CML is Ph1 positive or negative, for the latter form of the disease has a much poorer prognosis and requires a different therapeutic approach than the Ph1-positive disease. A system of "staging" of Ph1-positive CML is proposed, allowing a clear understanding of the type of therapy to be used and the results that can be expected from such therapy. The presence or absence of chromosomally normal cells plays a crucial role in this respect and points to the importance of diagnosing CML in its very early stages, when normal cells are usually present and leading to a much more favourable prognosis. The possible role of a missing Y in the leukemic cells and Ph1-translocations other than the standard one in the prognosis and clinical picture of CML is discussed.