Chronic lymphocytic leukemia (CLL) is a form of blood cancer in which too many lymphocytes are found in the body. Lymphocytes are a specific type of white blood cell. These abnormal lymphocytes accumulate in the bone marrow, blood, lymph tissue, and other organs of the body. The two main types of lymphocytes are B-cells and T-cells. Approximately 95% of CLL cases involve B-cells; the remainder are T-cell leukemias. The CLL FAQ deals primarily with B-cell CLL.

B-cell CLL is characterized by the relentless accumulation of malignant B-lymphocytes that have an abnormally extended life span and are unable to perform their proper functions. The progressive accumulation of these lymphocytes causes diminished production of normal bone marrow and blood cells.

Some patients have no symptoms at all and are said to be asymptomatic. This "smoldering" condition can continue for many years. Others may experience more rapid onset of symptoms and a more aggressive form of the disease. Symptoms may include enlarged lymph nodes, spleen, and liver. Severe anemia as well as low platelet counts, which can put patients at risk of bleeding, may also occur. Patients with CLL may also develop serious infections because of reduced numbers of infection-fighting cells known as neutrophils.