What is CLL?

Chronic lymphocytic leukemia (CLL) is a slow-growing blood cancer. This cancer occurs in a type of white blood cell known as B cells.1,2

In CLL, cancer cells grow out of control and live longer than healthy cells. These abnormal B cells crowd out healthy bone marrow and blood cells.2

What's happening in your body when you have CLL?

cll causes

Abnormal B cells crowd out healthy B cells.

What Causes CLL?

We know that CLL occurs when something changes in your genes and tells your B cells to grow out of control. But researchers are still trying to understand why genes change in the first place.1

cll body

CLL cells are most often found in the blood and bone marrow, but also in the lymph nodes, and spleen.

Where is CLL Found in the Body?

In CLL, cancer cells are found mostly in the blood and bone marrow, but also lymph nodes, and spleen.2

What’s the difference between CLL and small lymphocytic lymphoma (SLL)?

CLL and SLL are considered to be the same disease but with different origins. SLL cancer cells are found mainly in the lymph nodes.1

Want to learn more about CLL?

Watch this video to see what’s happening inside a CLL cell.

How Common Is CLL? 

When it comes to CLL, you may be asking, “Why me? Is there anything I could have done to avoid this?” At this time, there isn’t an answer since researchers are still trying to understand exactly what causes CLL.1

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Approximately 4.9 out of every 100,000 US adults are diagnosed with CLL each year.3

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Most people are diagnosed with CLL at around age 70.3

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CLL is slightly more common among men.3

A Change in Your Blood

Up to 60% of people with CLL are diagnosed before they experience any symptoms.4

Your diagnosis can come as a shock because you may not feel sick. CLL cells (abnormal B cells) build up slowly over time. This leads to changes in your blood, including changes in the number of these blood cells:2,3

Lower red blood cell counts (anemia)1

This can cause tiredness.

Lower platelet counts (thrombocytopenia)1

This can cause an increased tendency to bleed.

Lower counts of certain white blood cells (neutropenia)1

This can make it easier to get infections.

A routine blood test can detect these changes. This is often the first step leading to a diagnosis of CLL.1

You may not experience symptoms for years. Over time, signs and symptoms of CLL will appear. It’s important to keep your doctor updated when new symptoms appear or existing symptoms worsen. Symptom changes can mean your CLL is getting worse.1,2,5

What Are CLL Signs and Symptoms?

  • Tiredness
  • Chills
  • Fever
  • Night sweats
  • Infections
  • Unexpected weight loss
  • Swollen lymph nodes
  • Shortness of breath
  • Pain or a sense of "fullness" in the belly1,2

Be sure to reach out to your doctor if you experience chills, fever, or unexpected weight loss.2

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There’s More to Learn

Starting to understand the basics of CLL? Go a step further and learn all about CLL testing and staging.



  1. The Leukemia and Lymphoma Society. Chronic Lymphocytic Leukemia. White Plains, NY: The Leukemia and Lymphoma Society; 2014. Publication No. PS34 40M.
  2. Referenced with permission from the NCCN Guidelines for Patients®: Chronic Lymphocytic Leukemia V.2.2022. © NCCN Foundation®. 2022. All rights reserved. Accessed January 7, 2022. To view the most recent and complete version of the guideline, go online to NCCN.org/patients. NCCN makes no warranties of any kind whatsoever regarding their content, use or application and disclaims any responsibility for their application or use in any way.
  3. Cancer Stat Facts: Chronic Lymphocytic Leukemia (CLL). National Cancer Institute website. http://seer.cancer.gov/statfacts/html/clyl.html. Published April 2018. Accessed February 13, 2019.
  4. Kalil N, Cheson BD. Chronic lymphocytic leukemia. Oncologist. 1999;4(5):352-69.
  5. American Cancer Society. Chronic Lymphocytic Leukemia. American Cancer Society website. https://www.cancer.org/cancer/chronic-lymphocytic-leukemia.html. Updated May 10, 2018. Accessed January 7, 2022.