The final phase of chronic myeloid leukemia which has a high mortality rate. Myeloid leukemia is a form of cancer where the bone marrow makes too many myeloid cells (granulocytes and their precursors) in the bone marrow which accumulates in the blood and eventually invades various parts of the body. The three phases of the condition are the chronic phase, aggressive phase and finally the blast crisis. A blast crisis is occurs when over 30% of the cells in the blood or bone marrow are immature blood cells (blast cells). Patients in the final stage of leukemia are more prone to relapses following treatment. See also Blast crisis