Chronic lymphocytic leukemia (CLL) is the commonest type of leukemia seen in Western countries. It affects an older group of individuals than most other varieties of leukemia, and men more often than women, in a ratio of 2:1. The incidence of CLL is significantly increased in some families. In most instances, CLL is due to the overgrowth or accumulation of immunoglobulin producing B lymphocytes. Hypogammaglobulinemia is a common feature, and anomalous immunoglobulin components occur in 3 to 5% of patients. The early symptoms and signs of CLL include fatigue, reduced exercise tolerance, enlarged lymph nodes, and splenomegaly. Fever, weight loss, and impairment of bone marrow function, with anemia, bleeding and susceptibility to infection are characteristic of severe or advanced disease. In the great majority of patients, the disease can be controlled for 6 to 10 or more years with simple regimens using chlorambucil or cyclophosphamide, often in combination with prednisone. Radiotherapy and splenectomy are useful in some instances. The terminal phase of the disease is characterized by exacerbation or increasing severity of the leukemia and the development of opportunistic infections associated with immunodeficiency.