Chronic lymphocytic leukemia (CLL) is a typically slow-growing cancer which begins in lymphocytes in the bone marrow and extends into the blood. It can also spread to lymph nodes and organs such as the liver and spleen. CLL develops when too many abnormal lymphocytes grow, crowding out normal blood cells and making it difficult for the body to fight infection.

The term "chronic" means that the disease develops slowly. The abnormal lymphocytes take longer to develop and multiply. A disease like CLL, therefore, may take several years before it becomes serious. Comparatively, the progression of acute lymphocytic leukemia (ALL) is very quick. ALL may advance in a much shorter period.