Chronic lymphocytic leukemia, far from being simply a very chronic, very dull disease, has many facets which may help to illuminate the pathogenesis of a variety of immunologic aberrations. It appears likely that this is an accumulative disease of lymphocytes rather than one which is due to rapid proliferation, as was originally thought. The abnormal lymphocytes of the disease—with their various indications of metabolic but, more particularly, immunologic incompetency and their reduced ability to transform with both PNA and antigenic stimulation—eventually accumulate in large masses throughout the body. In the course of time, a host of disorders develop, including reduced immunity of both humoral and cellular varieties, autoimmunity, and extreme reactions to viruses and insects, numerous infections and wasting, finally leading to death. A striking response to corticosteroids is characteristic of the disease. On the other hand, large doses of x-ray or alkylating agents may result in violent and often fatal autoimmune reactions. Many future studies are indicated to demonstrate the nature of the metabolic defect in the lymphocyte and its pathogenesis, whether genetic or on some other basis. The possibility of modifying the tendency of the abnormal lymphocyte to accumulate, perhaps with the judicious use of corticosteroids, in association with other methods deserves intensive study.