Leukemia, chronic lymphocytic: The most common form of leukemia in adults, in which lymphocytes look fairly normal but are not fully mature and do not function correctly against infection. The malignant cells are found in blood and bone marrow, collect in and enlarge the lymph nodes, and may crowd out other blood cells in the bone marrow, resulting in a shortage of red cells (producing anemia) and platelets (producing bleeding and bruising). Abbreviated CLL. CLL is most common in people 60 years of age or older, and it progresses slowly. In the first stages of CLL, there are often no symptoms. As time goes on, more and more lymphocytes are made and symptoms begin to appear. Treatments may include chemotherapy, monoclonal antibody therapy, and bone marrow transplantation.