Skip to main content

Chronic Lymphocytic Leukemia (CLL)

Medically reviewed by Last updated on Feb 5, 2024.

What is Chronic Lymphocytic Leukemia (CLL)?

Harvard Health Publishing

Chronic lymphocytic leukemia (CLL)/small lymphocytic lymphoma (SLL) is a cancer that occurs when the bone marrow and/or lymph nodes make too many lymphocytes, a type of white blood cell. CLL/SLL usually grows slowly compared to other leukemias and lymphomas, and it may not cause symptoms for some time.

CLL is one of four main types of leukemia. Most people with CLL are middle-aged or older. The disease is very rare in children.

Normally, a person's immature blood stem cells develop into myeloid and lymphoid stem cells. The myeloid cells become mature blood cells: white blood cells, red blood cells, and platelets. Lymphoid stem cells develop into three types of infection-fighting lymphocytes:

In CLL/SLL, too many blood stem cells turn into abnormal lymphocytes. These cells do not function properly and they do not undergo normal removal from the blood stream. When the condition results primarily in a high number of circulating lymphocytes, it is referred to as CLL. Even in high numbers, they cannot fight infection as well as normal cells. As they pile up in the blood and bone marrow, they crowd out healthy white blood cells, red blood cells, and platelets. This can result in infection, anemia, and easy bleeding. CLL most often develops from B lymphocytes.

CLL begins in the bone marrow but then, as the abnormal lymphocytes multiply, they typically are released into the blood stream. In time, it can spread to other parts of the body that normally contain lymphocytes, such as the lymph nodes (in the neck, abdomen, and groin, under the arms, and around the collarbone), as well as the liver and the spleen. These tissues may then become enlarged. When the primary manifestation of the condition is enlarged lymphoid tissues, the condition is usually then referred to as small lymphocytic lymphoma (SLL).

Certain factors may increase a person's chance of developing CLL/SLL. These include

Keep in mind, however, that having one or more risk factors for CLL does not mean you will develop the disease.


People with CLL/SLL often have no signs or symptoms in the early stages of the disease. It may be discovered during a routine blood test or checkup, or when a blood test is done for another health problem.

Possible signs of CLL/SLL include

These symptoms can be associated with other cancers and many non-cancerous conditions. Tell your doctor if you experience any of them.

Symptoms of advanced CLL/SLL occur when there is a shortage of healthy blood cells:


Your doctor will ask you if you have symptoms such as fatigue, shortness of breath, or easy bruising. A physical exam will follow. Your doctor will check an enlarged spleen and lymph nodes.

Most often CLL/SLL can be diagnosed by a simple blood test. The first clue is an abnormally high number of lymphocytes. A pathologist can often confirm the diagnosis based on the presence of many abnormal-looking lymphocytes.

The leukemia cells in the blood can look a lot like low grade lymphoma cells. That's why it may be difficult to distinguish CLL from this type of cancer of the lymph nodes. But both conditions develop slowly, and the treatment is often similar.

Your doctor may order additional tests and procedures to diagnose CLL/SLL:

If you are diagnosed with CLL/SLL, you may be referred to a hematologist/oncologist, a doctor who specializes in cancers of the blood and bone marrow.

After you have been diagnosed, your doctor may do more tests to determine the extent of the CLL/SLL—that is, how far the cancer has spread. This step is called staging, and it helps your doctor plan your treatment. For example, your doctor may order a chest x-ray, MRI scan, CT scan, and/or ultrasound. These imaging tests create pictures of your body's insides. They can show abnormalities in organs or the spinal cord.

You may also have a lumbar puncture (spinal tap). During this procedure, a doctor uses a needle to withdraw some fluid from the spinal column. This may be done if he or she suspects that CLL/SLL cells have spread to the brain or spinal cord.

There are five stages of CLL/SLL, ranging from Stage 0 to Stage IV. Stage 0 means that there are too many lymphocytes in the blood, but that the patient has no other symptoms of leukemia. At the other end of the spectrum, Stage IV means that there are too many lymphocytes and too few platelets. The patient may have anemia (not enough iron, caused by too few red blood cells) and enlarged lymph nodes, liver, or spleen. These stages may be classified as low-, intermediate-, and high-risk.

Refractory CLL/SLL is cancer that does not get better with treatment.

Expected Duration

CLL/SLL usually grows slowly, and people can live for long periods, sometimes for decades, without treatment. However, some cases of CLL/SLL grow relatively quickly and require early treatment.


There is no known way to prevent CLL/SLL.


CLL/SLL generally can't be cured. The goals of treatment are to slow the buildup of CLL cells, maintain a patient's health, and help the patient avoid infection. This is a chronic disease and patients can live for prolonged periods of time with minimal to no decrease in their quality of life.

Decisions about treating CLL/SLL depend on several factors. These include

Patients with low-risk (stage 0) CLL/SLL that is not growing or growing slowly and who lack symptoms may wait to begin treatment—but continue with follow-up exams. This approach, called watchful waiting, allows the person to avoid the side effects of treatment until symptoms appear or change. However, problems caused by the disease, such as infection, are usually treated.

Patients with intermediate- and high-risk CLL/SLL (stages I–IV) who do not have symptoms may delay treatment while continuing to be observed. Treatment for faster-growing disease may include

Some people receiving CLL/SLL treatment may experience no side effects, while others face serious and long-lasting ones. Some possible side effects are

The disease and its treatment can lead to other complications. For example, you may develop another cancer in the future, or you may develop a condition in which your immune system attacks your own blood cells.

Several new drugs have recently been approved for CLL/SLL that potentially can induce and maintain a remission for a long period of time.

Treatment options

The following list of medications are in some way related to or used in the treatment of this condition.

View more treatment options

When To Call a Professional

Call your doctor if you have any symptoms of leukemia. If you are diagnosed with CLL/SLL, it's important to see a doctor who specializes in blood-related cancers. These specialists are called hematologists and oncologists.

Also, during a routine examination, your doctor may tell you that your white blood count is elevated and this may prompt a visit to a blood specialist.


There is no cure for CLL/SLL, but many people live for years with good health. The outlook depends on several factors. These include

Additional Information

National Cancer Institute (NCI)

American Cancer Society (ACS)

Leukemia & Lymphoma Society

Learn more about Chronic Lymphocytic Leukemia

Treatment options

Care guides

Further information

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.