Acute (sudden onset) lymphocytic leukemia (ALL) is also known as lymphoblastic or lymphoid leukemia. It is the most common form of leukemia in children.

Lymphocytes are a type of white blood cell that help fight infection. They are formed in the center cavity of certain bones, in a sponge-like tissue called marrow.

In this form of leukemia, the bone marrow makes too many lymphocytes. These lymphocyte cancer cells, also called blasts, are not normal. They do not fight infection well. They also crowd out other kinds of blood cells forming in the bone marrow, causing shortages. Because of this, there may not be enough oxygen-carrying red blood cells. There may also not be enough platelets, which help blood clot when needed.

Acute leukemia can occur over a short period of days to weeks. Symptoms are similar to those of other leukemias.