A malignant disease of the bone marrow and blood that progresses slowly and permits greater numbers of more mature, functional cells to be made. There are two major types of chronic leukemia: Chronic Myelogenous Leukemia (CML) and Chronic Lymphocytic Leukemia (CLL). Each has several subtypes. CLL is a common type of leukemia in older adults, accounting for approximately 7,500 new cases each year. CML strikes about 4,500 persons each year.