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Cancer starts when cells in the body begin to grow out of control. Cells in nearly any part of the body can become cancer, and can spread to other areas of the body. To learn more about how cancers start and spread, see What Is Cancer?

Leukemia is a type of cancer that starts in cells that form new blood cells. These cells are found in the soft, inner part of the bones called the bone marrow.

Chronic lymphocytic leukemia (CLL) is a cancer of lymphocytes, cells that become the white blood cells. In CLL, the leukemia cells build up in the bone marrow and spill out into the blood stream. They can also build up in lymph nodes, the spleen, liver, and other parts of the body. CLL is a slow growing leukemia, and it often takes years before it causes symptoms.

Lymphoma is another cancer that starts in lymphocytes. In one kind of lymphoma, small lymphocytic lymphoma, the cancer cell is the same as the leukemia cell in CLL. These diseases can have different signs and symptoms, but are treated the same way.

Acute lymphocytic leukemia (ALL) is another cancer that occurs in the cells that become lymphocytes. The cells in ALL look more abnormal and less mature than the cells in CLL. ALL is a much faster growing leukemia than CLL.

In addition to CLL and ALL, there are 2 other main types of leukemia:

These start in the early version of cells that become other kinds of blood cells (besides lymphocytes).

Different types of leukemia are treated differently.

Along with the main types, there are a few other, less common, types of leukemia. The information here is only about chronic lymphocytic leukemia (CLL) of adults. For information about other types of leukemia please see the separate American Cancer Society documents on these topics.

Last Medical Review: 01/30/2015
Last Revised: 02/23/2016