Chronic lymphocytic leukemia. B-cell chronic lymphocytic leukemia (B-CLL) is the most prevalent leukemia in the Western Hemisphere, accounting for ~25% of all leukemia's. It represents a monoclonal expansion of small, long-lived, apparently slowly dividing CD5+ B cells. Because of the low proliferative index and a presumed uniform proliferative rate of B-CLL cells in vivo (a fact not yet tested or documented), B-CLL appears to be primarily a disease of accumulation rather than proliferation.