Chronic lymphocytic leukemia (CLL) is a type of cancer that affects the blood and bone marrow, which is the spongy tissue inside bones where blood cells are made. It is the most common form of leukemia in adults, and it typically progresses more slowly than other types of leukemia. The term "lymphocytic" in CLL comes from the cells affected by the disease, which are a group of white blood cells called lymphocytes that help the body fight infection. CLL most commonly affects older adults, but it can affect people starting at age 30.

Doctors arent certain what starts the process that causes CLL, but they know that something happens to cause changes (mutations) in the DNA of blood-producing cells. These changes tell the blood cells to produce abnormal, ineffective lymphocytes. CLL may cause complications such as swollen lymph nodes, feeling tired, pain or a sense of fullness under the ribs, and low numbers of all three blood cell counts.

CLL can progress either slowly or quickly depending on the form it takes. There are two different forms of CLL: one that grows slowly and may not require immediate treatment, and another that grows at a faster rate and requires treatment right away. CLL patients have a number of effective treatment options available to them, including chemotherapy, monoclonal antibodies, and targeted therapy drugs. While healthcare providers cant cure CLL, they can provide treatments to eliminate CLL symptoms and signs of disease, putting the disease into remission. Some people with CLL live for many years.