Intravenously administered H³TdR was used to study peripheral blood leukocyte kinetics of ten patients with CLL in relapse and one in partial remission. Two distinct patterns were found. The first pattern indicates the presence of a small, proliferating population of lymphocytes which is present in the blood for a relatively brief time. This pattern was found in patients with no organomegaly and with relatively lower peripheral lymphocyte counts. In the second pattern almost all of the cells present were non-proliferating and long-lived. The patients with the latter pattern had organomegaly. These studies support the concept that there is a progressive widespread accumulation of long-lived lymphocytes in CLL often leading to splenic pooling and, in some cases, hypersplenism.