An advanced phase of chronic myelogenous leukemia. It is characterized by: 1. the presence of blasts in the peripheral blood or bone marrow that are at least 20% of the peripheral blood white cells or of the nucleated cells in the bone marrow respectively, or 2. an extramedullary proliferation of blasts, and/or 3. when there are large aggregates and clusters of blasts in the bone marrow biopsy specimen (adapted from WHO, 2001).