(immunohistochemical, B cell origin, progression to an acute aggressive form of non-Hodgkin's lymphoma), the committee has reviewed CLL separately from the other leukemias; the committee's review of CLL is presented in the next section.

The incidence of chronic myeloid leukemia (CML) increases steadily with age in people over 30 years old. Its lifetime incidence is roughly equal in whites and blacks and is slightly higher in males than in females. Among people in the age groups that characterize most Vietnam veterans, CML accounts for about one-fifth of cases of leukemia. It is associated with an acquired chromosomal abnormality known as the Philadelphia chromosome; exposure to high doses of ionizing radiation is a known risk factor for this abnormality.

Little is known about the risk factors associated with other forms of leukemia. However, two human retroviruses have been linked to human leukemias: HTLV-1 appears to cause adult T-cell leukemia or lymphoma, whereas the data linking HTLV-2 to hairy-cell leukemia are less definitive.