Leukemia develops when a leukocyte undergoes a transformation into a malignant cell, one capable of uncontrolled growth. Leukemia cells begin to multiply in the marrow, and as they do so they crowd out the normal blood cells, those that carry oxygen to the body's tissues, fight infections, and help wounds heal by clotting the blood. Leukemia can also spread from the marrow to other parts of the body, including the lymph nodes, brain, liver, and spleen.

Leukemia is ten times more common among adults than among children. Leukemias are evenly split between the acute and chronic forms, but among children one form, acute lymphocytic leukemia, accounts for about two-thirds of cases. Acute myeloid leukemia and chronic lymphocytic leukemia are the most common types in adults.

Blood cell development begins in the marrow with the formation of stem cells. These primitive cells are capable of developing into any kind of blood cell. A malignant transformation can happen at any stage of blood cell development. The leukemia cells that result carry many characteristics of the cell from which they began. Most leukemias fall into one of two general groups: myeloid leukemia and lymphocytic leukemia.

Physicians also classify leukemias according to whether they are acute or chronic. In acute leukemias, the malignant cells, or blasts, are immature cells that are incapable of performing their immune system functions. The onset of acute leukemias is rapid, and, in most cases, fatal unless the disease is treated quickly. Chronic leukemias develop in more mature cells, which can perform some of their duties but not well. These abnormal cells also increase at a slower rate, so the disease develops more slowly than in acute leukemia, and in many cases is more difficult to cure.