Chronic lymphocytic leukemia (CLL) is the most common leukemia in adults. It's a type of cancer that starts in cells that become certain white blood cells (called lymphocytes) in the bone marrow. The cancer (leukemia) cells start in the bone marrow but then go into the blood.

In CLL, the leukemia cells often build up slowly. Many people don't have any symptoms for at least a few years. But over time, the cells grow and spread to other parts of the body, including the lymph nodes, liver, and spleen.