Phases of CML

CML develops slowly. There are three phases of CML:

  • the chronic phase
  • the accelerated phase
  • the blast phase.

Your phase depends on the number of immature cells (blast cells) in your blood and bone marrow, and your symptoms. Most people are diagnosed when CML is in the chronic phase.

The chronic phase

CML develops very slowly in the chronic phase and is often stable for a long time. The chronic phase is sometimes called the stable phase. There may be no symptoms and most people lead a normal life.

People in this phase rarely need to go into hospital. They can have treatment as an outpatient and it doesn’t usually cause many side effects. They will have regular blood tests to check how well they are responding to treatment.

Most people who have treatment in the chronic phase have their leukaemia well-controlled and won’t have any symptoms. If they keep taking their treatment, their leukaemia may be kept under control for many years and potentially for the rest of their life.

The accelerated phase

In some people, CML does not respond as well to treatment. In a small number of people, the leukaemia may progress from the chronic phase to the accelerated phase. Occasionally people may be diagnosed with CML in the accelerated phase.

In this phase, there are more blast cells in the blood and bone marrow. You may also develop symptoms such as tiredness, weight loss, bone pain, sweating and a high temperature at night. If you feel unwell or develop new symptoms, let your doctor know.

The treatment for the accelerated phase may be more intensive than for the chronic phase, and you may need to spend some time in hospital. Your specialist can tell you more about this.

The blast phase

After some time in the accelerated phase, the leukaemia may transform into the blast phase. Occasionally people may be diagnosed in the blast phase or progress straight to this from the chronic phase. 

The blast phase is like an acute leukaemia. In this phase, there are even more blast cells. This phase is sometimes called blast crisis.


Remission (a complete response) means the blood and bone marrow go back to normal following treatment. There are different levels of response.


Relapse means the leukaemia cells have come back after a time in remission.

Back to Understanding your diagnosis

Treatment overview

Your doctor will discuss treatment options with you. These will depend on the phase of the leukaemia and your general health.