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Chronic lymphocytic leukemia (CLL), also called B-cell chronic lymphocytic leukemia, is typically a slow-growing cancer of a type of white blood cells called lymphocytes. Normal lymphocytes help your body fight infections. In CLL, the lymphocytes do not develop properly and don’t fight infections very well. These diseased cells (leukemia cells) eventually crowd out the bone marrow, preventing it from making healthy white blood cells, red blood cells, and platelets that the body needs.

CLL mostly affects older adults, and it is very rare in children and young adults. In the United States, about 15,000 people are diagnosed with CLL each year.1 Doctors do not know what causes CLL.

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