Chronic myelogenous leukemia (CML) accounts for 15% of adult leukemias. In 2005, an estimated 4,600 cases will be diagnosed, and 850 patients will die of the disease. The median age of disease onset is 53 years; however, CML occurs in all age groups, with an increasing proportion of younger patients in more recent series. ML progresses from a chronic phase to a rapidly fatal blastic phase, generally over 3 to 5 years. The blast phase is often preceded by a transition period, called the accelerated phase, which is marked by the acquisition of new cytogenetic abnormalities in 50% to 80% of patients. Several definitions of the accelerated phase of the disease are summarized. © Journal of the National Comprehensive Cancer Network.