CLL is the most common form of leukemia in the western world. It accounts for approximately 25 to 30 percent of all leukemias. CLL occurs more frequently in older individuals but is seen increasingly in younger patients as well. It is twice as common in men as in women. Approximately 1.8 to 3 new cases per 100,000 Americans are diagnosed each year. In the age group 35 to 59, the annual incidence rises to 5.2 new cases per 100,000 Americans, and in the age group 80 to 84, it rises further to 30.4 new cases per 100,000 Americans.