What is myelofibrosis?

Myelofibrosis (MF) is pronounced my-eh-lo-fy-bro-sis. Myelo means bone marrow and fibrosis relates to the development of fibrous or scar tissue. The scar tissue builds up inside your bone marrow and blood cells can't develop properly. 

Myelofibrosis can affect people of any age. But it usually affects middle-aged and older people. The average age at diagnosis is 65 years.

Pre-fibrotic or early stage primary mylofibrosis means you have changes to your bone marrow but you do not have any bone marrow scarring.

Primary myelofibrosis

People who have no history of problems with their bone marrow can get MF. This is called primary myelofibrosis (PMF) or sometimes chronic idiopathic myelofibrosis. 

Secondary myelofibrosis

Secondary myelofibrosis is where the condition develops in people who have other bone marrow disorders such as polycythaemia vera or essential thrombocythaemia

Myeloproliferative neoplasms

Myelofibrosis is a type of blood disorder called a myeloproliferative neoplasm. These are conditions that cause an increase in the number of blood cells.

The World Health Organisation (WHO) classes all myeloproliferative neoplasms as blood cancers. This is because the bone marrow is producing blood cells in an uncontrolled way. But many people with myeloproliferative neoplasms feel well and only need gentle treatment. The disorders often develop slowly and progress slowly. Or they can remain stable for a while.

The bone marrow and blood cells

The bone marrow is the soft inner part of our bones that makes blood cells. All blood cells start from the same type of cell, called a stem cell. The stem cell makes immature blood cells.

The immature cells go through various stages of development before they become fully developed blood cells and are released into the blood as:

  • red blood cells to carry oxygen around our bodies
  • white blood cells to fight infection
  • platelets to help the blood clot

The diagram shows how the various types of cell develop from a single blood stem cell.

A simplified diagram showing how blood cells are made

What happens in myelofibrosis?

In people with myelofibrosis, the bone marrow is too active, then scar tissue builds up (fibrosis). Because of this, blood cells are not made properly. The bone marrow gradually produces fewer blood cells. As the number of new blood cells fall in the bone marrow, the liver and spleen try to make more blood cells. But they are not as good at making them as the bone marrow, so you might have too few red blood cells (anaemia).

The spleen can become bigger (enlarged). Instead of releasing the red blood cells into the blood it keeps hold of them. This might cause discomfort in your tummy or make you feel full when you eat.

As myelofibrosis develops, the bone marrow also makes fewer white blood cells and platelets. 

Myelofibrosis and acute myeloid leukaemia (AML)

How myelofibrosis develops in people can vary. Treatment can control myelofibrosis for many people for some time.

Some people develop more serious problems, such as heart disease and infections that might be difficult to control.

Around 10 to 20 out of every 100 people (around 10 to 20%) go on to develop acute myeloid leukaemia (AML). AML is a type of blood cancer that develops from young white blood cells called granulocytes or monocytes in the bone marrow. 

In this situation, your doctor would talk to you about starting treatment for AML.

Risks and causes

Some research suggests that exposure to the chemical benzene may increase your risk of developing myeloproliferative neoplasms. You are most likely to come into contact with benzene through work. It is used in the rubber industry and is one of the chemicals in petrol.

More than 55 in 100 people (more than 55%) with myelofibrosis have a change in a gene Open a glossary item called JAK2. The JAK2 gene makes a protein that controls how many blood cells the stem cells make. 

Up to 35 in 100 people (up to 35%) have a change in the CALR gene.

When the JAK2 or CALR gene becomes mutated Open a glossary itemyour bone marrow may not function correctly. This means scar tissue can build up in your bone marrow.

Some people with myelofibrosis have a history of essential thrombocythaemia or polycythaemia vera.


Myelofibrosis usually develops slowly and doesn’t cause symptoms at first. Some people find out about their myelofibrosis when having a blood test for something else.

As the condition progresses it causes symptoms. These might include:

  • tiredness and shortness of breath - due to low numbers of red blood cells
  • bleeding and bruising easily - due to low numbers of platelets
  • pain and discomfort in the tummy (abdomen) due to enlarged spleen and liver
  • bone pain
  • gout - you might have painful, stiff or swollen joints
  • loss of appetite and weight loss
  • fever and night sweats
  • very itchy skin

Myelofibrosis is rare, so if you have any of these symptoms it is more likely to be something else. But it is still important to contact your doctor to find out what is causing them.

Last reviewed: 
27 Jul 2020
Next review due: 
27 Jul 2023
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