There are many types and subtypes of leukemia. In general, the disease is first classified according to how quickly it progresses. The second classification is based on the type of white blood cell from which the leukemia develops. The abnormal cells may be considered mature or immature depending on the cell type. Leukemia can develop in two forms of blood cells, lymphocytes and myeloid cells, and at two different rates of progression: acute and chronic.

The main terms used to describe leukemia include:

• Acute. Acute leukemias are leukemias in which the cancer cells develop from early cells known as blast cells. The cells grow quickly but they fail to mature properly. This form grows rapidly because the cells divide often and continue to divide longer than normal blast cells. Although the white blood cell count is frequently high in these forms of leukemia, it can also be normal or low. Most cases of leukemia in children are acute, requiring aggressive and immediate treatment.
  
  
• Chronic. Chronic leukemias are leukemias in which the cancer cells arise from more mature cells, but the cells are not entirely normal. The cells live abnormally long and begin to increase. As a result, a patient’s white blood cell count is almost always high at diagnosis. These forms of leukemia are usually slow-growing.
  
  
• Lymphocytic. Leukemias of this type develop from lymphoblasts or lymphocytes in the bone marrow, which form lymphatic tissue. A main component of the immune system, lymphatic tissue is found throughout the body, and includes the lymph nodes and vessels, spleen and tonsils.
  
  
• Myelogenous. Also known as myeloid or myelocytic leukemias, leukemias of this type develop from myeloid cells. The myeloid cells include cells that eventually develop into red blood cells, white blood cells and platelet-producing cells.
  

By examining these factors, most forms of leukemia can be classified into four main types. These include:

• Acute lymphocytic leukemia (ALL)
  
• • Also known as acute lymphoblastic leukemia, the earliest lymphocyte cell
    
    
  • According to the American Cancer Society (ACS), ALL will account for about 5,200 new cases of leukemia in 2007
    
    
  • Two-thirds of all cases of ALL occur in children
    
    
  • Typically occurs between ages 3 and 6
    
    
  • Most common leukemia in children
    
    
  • Treatment is primarily chemotherapy, focusing on brain and spinal cord, where it commonly spreads
    
    
  • Several types of ALL, each with a different cure rate
    
    
  • Prognosis is greatly affected by age
    
    
• Acute myeloid leukemia (AML)
  
• • Also known as acute myelogenous leukemia
    
    
  • According to the ACS, AML will account for approximately 13,410 new cases of leukemia in 2007
    
    
  • Roughly 90 percent of AML cases occur in adults
    
    
  • Based on the appearance of the cells, AML is divided into a variety of subtypes known as M1 – M7
    
    
  • Chemotherapy is the main treatment method for most subtypes
    
    
  • Prognosis depends on the subtype of the disease
    
    
• Chronic lymphocytic leukemia (CLL)
  
• • Involves mature-looking lymphocytes
    
    
  • Occurs mainly in older adults, with the average age in the 60s
    
    
  • According to the ACS, CLL will account for approximately 15,340 new cases of leukemia in 2007
    
    
  • Main treatment method is chemotherapy and/or biological therapy
    
    
• Chronic myelogenous leukemia (CML)
  
• • Also known as chronic myeloid leukemia
    
    
  • Usually occurs in middle-aged patients, with the average age in the 50s
    
    
  • Involves myelocytic cells
    
    
  • According to the ACS, CML will account for approximately 4,500 new cases of leukemia in 2007
    
    
  • Treatment methods include the drugs imatinib mesylate, interferon-alpha and chemotherapy
    

In addition to the four main types of leukemia, there are also a number of rare types. Rare leukemias, including hairy cell leukemia, account for almost 4,700 new cases of leukemia each year.

Studies have shown the best predictor for recovery outcome of leukemia may be based on the abnormality of the chromosome rather than the actual leukemia subtype. For this reason, cytologic studies may be conducted to determine the chromosome damage in addition to the type.