[Skip to Content]

Juvenile Myelomonocytic Leukemia (JMML)

Reviewed by: Edward A. Kolb, MD

What Is Juvenile Myelomonocytic Leukemia?

Juvenile myelomonocytic leukemia (JMML) is a rare childhood cancer. It happens when immature blood cells (called blasts) make too many myelocytes and monocytes (two types of white blood cells). These myelocytes, monocytes, and blasts crowd the normal cells in the bone marrow and other organs in the body, causing the symptoms of JMML.

Who Gets Juvenile Myelomonocytic Leukemia?

Juvenile myelomonocytic (mye-eh-low-mah-nuh-SIT-ik) leukemia usually happens in children younger than 4 years old.

What Are the Signs & Symptoms of Juvenile Myelomonocytic Leukemia?

JMML develops over time. A child may have very few symptoms at first because they can take months to develop.

A child with juvenile myelomonocytic leukemia might:

  • develop anemia, which is when the bone marrow doesn't make enough red blood cells
  • get infections often (such as or tonsillitis) because the white blood cells don't fight germs as usual
  • feel tired and weak because of anemia and infections
  • have swollen lymph nodes from collected cancer cells
  • have fevers from infections
  • have easy bruising or bleeding because the bone marrow doesn't make enough platelets (cells that help clotting)
  • have bone and joint pain because the bone marrow is full of abnormal cells
  • have belly pain and swelling if abnormal blood cells build up in organs like the kidneys, liver, and spleen

What Causes Juvenile Myelomonocytic Leukemia?

Many children diagnosed with juvenile myelomonocytic leukemia have a change in their genes (a mutation). The change happens in monocytes (MAH-nuh-sites), which are part of the immune system.

Some medical conditions — such as neurofibromatosis type 1 and Noonan syndrome — can make a child more likely to develop JMML.

How Is Juvenile Myelomonocytic Leukemia Diagnosed?

Doctors use special tests when they suspect leukemia. These include:

  • Blood tests. Tests such as a complete blood count, liver and kidney function panels, and blood chemistries can give important information about the number of normal blood cells and how well the organs are working. The shapes and sizes of the blood cells are checked with a microscope.
  • Imaging studies. These may include an X-ray, CT scan, MRI, or ultrasound. These can see whether there's a mass of leukemia cells in the chest that could affect breathing or blood circulation. They also can rule out other possible causes of a child's symptoms.
  • Bone marrow aspiration and biopsy. The doctor puts a needle into a large bone, usually the hip, and removes a small amount of bone marrow. Then the lab does these tests on the bone marrow sample:
    • Flow cytometry tests. Doctors look at the cancer cells and figure out the type of leukemia. This is important because treatment differs depending on the type.
    • Genetic tests. By looking at the blood or bone marrow, doctors check for changes in the genes. The changes can help doctors figure out the best treatment.
  • Spinal tap (lumbar puncture). The doctor uses a small hollow needle to remove a tiny amount of cerebrospinal fluid (the fluid surrounding the brain and spinal cord) for exam in a lab.
  • Tissue typing or HLA (human leukocyte antigen) typing. If a child needs a stem cell transplant (bone marrow transplant), this test helps doctors find a suitable stem cell donor. It works by comparing the proteins on the surface of a child's blood cells with the proteins on a potential donor's cells. The more HLA markers a child and donor share, the greater the chances that a transplant will be successful.

How Is Juvenile Myelomonocytic Leukemia Treated?

Chemotherapy (the use of drugs to kill cancer cells) may be used to temporarily control juvenile myelomonocytic leukemia. 

But the most effective treatment is a stem cell (bone marrow) transplant. To do this, doctors:

  • Give high-dose chemotherapy to destroy cancer cells and normal bone marrow and immune system cells.
  • Put healthy donor stem cells into the body. The new stem cells can rebuild a healthy blood supply and immune system.

Researchers are looking into the use of alternative treatments, such as:

  • Molecular-targeted therapies: These medicines slow the growth of cancer cells by blocking certain molecules or proteins that help cancer cells grow.
  • Immunotherapy: These medicines mark cancer cells so that the body's immune system can find them and remove them from the body.

What Else Should I Know?

Having a child being treated for cancer can be stressful for any family. But you're not alone. To find support, talk to your doctor or a hospital social worker. Many resources are available to help you get through this difficult time. You also can find information and support online at:

Reviewed by: Edward A. Kolb, MD
Date reviewed: May 2019