Twenty-five patients with Ph CML who eventually developed a blast crisis were karyotyped at regular intervals in order to correlate the evolution of abnormal clones with clinical changes. Persistent new clones appeared in the chronic phase, prior to either transition or transformation, or in the acute phase (the latter, particularly, where transformation was slow). In many patients, chromosome changes accumulated within a single abnormal clone as the disease progressed. In others, particularly where the simple Ph cell line was slow to be supplanted, new clones appeared from the remaining Ph cells at the time of transformation, perhaps reflecting the inability of certain chromosome abnormalities to coexist. We suggest that the occurrence of additional chromosome abnormalities is not reliable evidence of acute transformation, but that the nature and subsequent behavior of abnormal clones may provide more valuable indications.