Chronic leukemias — chronic myeloid leukemia and chronic lymphocytic leukemia — occur mostly in adults and are extremely rare in children and young persons. Chronic lymphocytic leukemia has the highest familial incidence of the leukemias. Chronic myeloid leukemia causes rapid growth of the blood-forming cells (myeloid precursors) in the bone marrow, peripheral blood, and body tissues, whereas chronic lymphocytic leukemia causes a slow increase in the number of B lymphocytes in the bone marrow and also can affect the lymph nodes and other organs. Among the risk factors for chronic leukemias are occupational exposure to benzene, gases, solvents, paints, and other substances as well as exposure to radiation. Each type of leukemia has specific characteristics and treatments, which vary according to the stage of the disease. Among the therapeutic options for patients with chronic forms of leukemia are chemotherapy, biologic therapy, radiation, and stem cell or bone marrow transplantation.