A risk factor [2] is anything that increases a person's chance of developing cancer. Although risk factors often influence the development of cancer, most do not directly cause cancer. Some people with several risk factors never develop cancer, while others with no known risk factors do. However, knowing your risk factors and talking about them with your doctor may help you make more informed lifestyle and health care choices.

The cause of CLL is unknown. There is no evidence showing that exposure to radiation, chemicals, or chemotherapy increases a person's risk of developing CLL. The following factors may raise a person's risk of developing CLL:

Family history. In some families, more than one close relative may develop CLL or some other lymph-related cancer; however, this is uncommon. About 10% of newly diagnosed patients with CLL will have another family member with a similar disease of lymphocytes. In order to learn more about families with a history of CLL, there is a registry of such families [3] at the National Cancer Institute.

Age. CLL is most common in older adults, is rare in young adults, and hardly ever develops in children. About 90% of people diagnosed with CLL are older than 50. The average age at diagnosis is close to 70 years.

Gender. Men develop CLL more often than women.

Ethnicity. B-cell CLL is more common in people of Russian and European descent, and hardly ever develops in people from China, Japan, or Southeast Asian countries. The reason(s) for this geographic difference is not known.

Agent Orange. The U.S. Department of Veterans Affairs lists CLL as a disease associated with exposure to Agent Orange [4], a chemical used during the Vietnam War.