Cancer of the blood or blood-forming organs.  People with leukemia often have a noticeable increase in white blood cells (leukocytes).

Leukemia begins in the bone marrow when abnormal cells multiply out of control to such an extent that normal blood cells are unable to develop.

Leukemia starts in the bone marrow—the soft, spongy material inside bones where blood cells develop from stem cells. Leukemia most often affects the white blood cells, which protect the body against infection.

The disease begins when normal development of stem cells into white blood cells goes wrong, creating abnormal white blood cells. Eventually the abnormal white cells overtake other types of blood cells, including red blood cells, which carry oxygen to the body tissues; and platelets, which enable the blood to clot. Leukemia therefore interferes with the blood's ability to clot and carry oxygen.

The abnormal white cells may also spread to other parts of the body, including the lymph nodes, skin, spleen, liver, brain, and spinal cord.

Leukemia is not one disease but a complex group of diseases. There are four main types of leukemia: acute lymphocytic leukemia (ALL), acute myeloid (or myelogenous) leukemia (AML), chronic lymphocytic leukemia (CLL), and chronic myeloid leukemia (CML).