There are several types of leukemia which differ dramatically in prognosis, treatment, and symptoms. Leukemia is either acute or chronic and usually arises in either of the two main types of white blood cells--lymphoid cells or myeloid cells. When leukemia affects the lymphoid cells, it is called lymphocytic leukemia. When myeloid cells are affected, the disease is called myeloid or myelogenous leukemia. The most common types of leukemia are:

Acute lymphocytic leukemia (ALL) - ALL is the most common type of leukemia in young children. This disease also affects adults, especially those age 65 and older. ALL can be divided into subcategories by the type of cell or the genetic damage found in the cells.

Acute myeloid leukemia (AML) - AML occurs in both adults and children. This type of leukemia is sometimes called acute nonlymphocytic leukemia (ANLL). AML can be subdivided into categories based on the genetic damage found in the cells.

Chronic lymphocytic leukemia (CLL) - CLL most often affects adults over the age of 55. It sometimes occurs in younger adults, but it almost never affects children.

Chronic myeloid leukemia (CML) - CML occurs mainly in adults. A very small number of children also develop this disease.

These are not the only types of leukemia, but they are the most common forms of the disease.