The types of leukemia also can be grouped based on the type of white blood cell that is affected. Leukemia can start in lymphoid cells or myeloid cells. Leukemia that affects lymphoid cells is called lymphoid, lymphocytic, or lymphoblastic leukemia. Leukemia that affects myeloid cells is called myeloid, myelogenous, or myeloblastic leukemia.

There are four common types of leukemia:

  • Chronic lymphocytic leukemia (CLL): CLL affects lymphoid cells and usually grows slowly. It accounts for more than 15,000 new cases of leukemia each year. Most often, people diagnosed with the disease are over age 55. It almost never affects children.
  • Chronic myeloid leukemia (CML): CML affects myeloid cells and usually grows slowly at first. It accounts for nearly 5,000 new cases of leukemia each year. It mainly affects adults.
  • Acute lymphocytic (lymphoblastic) leukemia (ALL): ALL affects lymphoid cells and grows quickly. It accounts for more than 5,000 new cases of leukemia each year. ALL is the most common type of leukemia in young children. It also affects adults.
  • Acute myeloid leukemia (AML): AML affects myeloid cells and grows quickly. It accounts for more than 13,000 new cases of leukemia each year. It occurs in both adults and children.

Another type of leukemia is hairy cell leukemia. Hairy cell leukemia is a rare type of leukemia in which abnormal B-lymphocytes (a type of white blood cell) are present in the bone marrow, spleen, and peripheral blood. When viewed under a microscope, these cells appear to be covered with tiny hair-like projections.