Chronic myeloid leukaemia (CML) Chronic myeloid leukaemia (CML) is a serious condition, but if it's caught in the early stage, it's possible to do well on treatment for many years. 'Chronic' means CML can take a long time to develop, and 'myeloid' tells you it affects a particular group of white blood cells. CML develops because your body's process for making white blood cells goes wrong. You can learn more about white blood cells and this process, called haematopoiesis in our general information about leukaemia. What are the symptoms of CML? Many people with CML don't feel unwell at first. The symptoms of CML can include: tiredness night sweats swelling in your abdomen pale skin breathlessness fever bruises unusual bleeding frequent infections aching arms and legs What causes CML? We don't know what causes CML. It's most likely to affect people aged between 60 and 65. It happens more often in men than in women. Most people with CML have what’s called 'the Philadelphia chromosome' in their cancer cells. Despite the way its sounds, CML is not passed down through the family. How is CML diagnosed? Because it doesn't always cause symptoms at first, CML is often picked up when you have a blood test for another reason. If CML is suspected, you'll be offered more blood tests and a bone marrow biopsy to confirm the diagnosis. Later on, you'll be offered these tests regularly to monitor how well your treatment is working. You can read more in our information about tests for leukaemia. What are the phases of CML? CML can progress over time, which happens in three phases: Chronic phase The cancer is progressing slowly. The outlook is best in this phase and with treatment, CML does not go any further for most people. Accelerated phase The cancer progresses more quickly. The bone marrow is making too many immature white blood cells (called blasts), but the number is still relatively small. Blast phase or blast crisis The cancer becomes more aggressive and transforms into an acute leukaemia. At this stage, the bone marrow becomes filled with immature white blood cells (blasts) in much greater numbers and there are also more blasts in the blood. This phase can progress more quickly and you can feel quite unwell.