About phases of chronic myeloid leukaemia (CML)

Chronic myeloid leukaemia (CML) develops slowly. There are three possible phases of CML:

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

Doctors know the phase of CML you are in from:

Most people are diagnosed when CML is in the chronic phase.

Chronic phase

When CML is in the chronic phase, there may be no symptoms and most people lead a normal life. People usually have treatment by taking tablets at home. They have regular blood tests to check how well treatment is working.

For most people, the leukaemia can be well controlled for as long as they continue to take treatment.

In the chronic phase, less than 1 in 10 (10%) of the blood cells in the blood or bone marrow are blast cells.

Accelerated phase

Sometimes CML does not respond very 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 or 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.

Blast phase

In some people who have CML in the accelerated phase, the leukaemia may transform into the blast phase. Occasionally people may be diagnosed with CML in the blast phase or progress straight to it from the chronic phase.

The blast phase is like an acute leukaemia. In this phase, more than 2 in 10 (20%) of the blood cells in the blood or bone marrow are blast cells. The blast phase is sometimes called blast crisis.


Relapse means the leukaemia cells have come back after a time in remission (where there are no signs of leukaemia cells).

About our information


  • References

    Below is a sample of the sources used in our chronic myeloid leukaemia (CML). If you would like more information about the sources we use, please contact us at cancerinformationteam@macmillan.org.uk

    European Leukemia Net. Recommendations for the management of chronic myeloid leukemia. 2013.

    Hoffbrand V, and Moss P. Hoffbrand’s essential haematology. 7th edition. 2016.

    National Institute for Health and Care Excellence. Leukaemia (chronic myeloid) – dastatinib, nilotinib and standard dose imatinib for the first-line treatment of chronic myeloid leukaemia (part review of technology appraisal guidance 70). April 2012.

    National Institute for Health and Care Excellence. Technology appraisal guidance. 401/426/425.

    DeVita V, Lawrence T and Rosenberg S. 2016. Lymphomas and leukemias. From Cancer: principles and practice of oncology.


  • Reviewers

    This information has been written, revised and edited by Macmillan Cancer Support’s Cancer Information Development team. It has been reviewed by expert medical and health professionals and people living with cancer. It has been approved by Senior Medical Editor, Dr Anne Parker, Consultant Haematologist.

    Our cancer information has been awarded the PIF TICK. Created by the Patient Information Forum, this quality mark shows we meet PIF’s 10 criteria for trustworthy health information.