To help doctors plan treatment and predict prognosis (chance of recovery), CML is divided into three different phases: chronic, accelerated, or blastic.

Chronic phase. The blood and bone marrow contain less than 5% blasts. Blasts are the most immature blood cells, and in CML, these cancerous blast cells cannot mature. This phase can last for several years, although without effective treatment the disease can progress to the accelerated or blast phases (see below). About 90% of people have chronic phase CML when they are diagnosed. Some people with chronic phase leukemia have symptoms when they are diagnosed and some do not; most symptoms go away once treatment begins.

Accelerated phase. In the accelerated phase, there are more than 5%, but less than 30% blasts in both the blood and bone marrow. These cells often have new cytogenetic changes in addition to the Philadelphia chromosome, because of additional DNA damage and mutations (changes) in the CML cells.

Blast phase (also called blast crisis). In the blast phase, there are more than 30% blasts in the blood or bone marrow. It develops when the CML cells begin behaving like acute leukemia. Patients in blast crisis often have a fever, an enlarged spleen, weight loss, and generally feel unwell.

Recurrent CML. Recurrent CML is SML that comes back after treatment.

Without effective treatment, patients with CML in chronic phase will move into blast crisis in an average of approximately five years after diagnosis. Patients who have more blasts or an increased number of cells called basophils (a special type of white blood cell), chromosome changes in addition to the Philadelphia chromosome, high numbers of white blood cells, or a very enlarged spleen often experience blast crisis sooner.