Leukemia starts in the tissue that forms blood. The disease develops when bone marrow makes abnormal white blood cells.

Most often, leukemia is a cancer of the white blood cells, but some leukemias start in other blood cell types. Leukemia cells don't die when they should and may crowd out normal white blood cells, red blood cells and platelets. The condition makes it hard for normal blood cells to do their work.

After it starts, leukemia often moves quickly into the blood where it can spread to other parts of the body such as the lymph nodes, spleen, liver, central nervous system (brain and spinal cord) and other organs. In contrast, other types of cancer can start in these organs and then spread to the bone marrow (or elsewhere). Those cancers are not leukemia.
Leukemia is the most common cancer in children and adolescents. It accounts for about one out of three cancers in children.

For more information on leukemia, visit the National Cancer Institute website.