When chronic lymphocytic leukemia (cll) was last reviewed in the Journal, it was considered a homogeneous disease of immature, immune-incompetent, minimally self-renewing B cells, which accumulate relentlessly because of a faulty apoptotic mechanism. In the past decade, these views have been transformed by a wealth of new information about the leukemic cells. CLL is now viewed as two related entities, both originating from antigen-stimulated mature B lymphocytes, which either avoid death through the intercession of external signals or die by apoptosis, only to be replenished by proliferating precursor cells.