In two untreated patients with progressive CLL, quantitative 14C autoradiography of lymph nodes and, subsequently, continuous infusion of [3H] thymidine over eight and nine days, respectively, were performed in order to analyse the lymph node cell kinetics. Simultaneously, the turnover of labelled lymphocytes in the peripheral blood was evaluated. From another CLL patient a regional lymph node was removed 6 h after an intralymphatic flash injection of [3H] thymidine and sectioned for autoradiographic study of the distribution of labelled cells within the lymph node tissue. While the durations of DNA synthesis were found to be normal, the labelling indices were reduced. The relative cell production rate was far lower than normal. Very small growth fractions were calculated, amounting to less than 1% in one patient, and to 2.4% in the other. The distribution of labelled cells in the lymph nodes was focal, which supports the finding of low growth fractions. According to the present data, CLL is a disorder in which a very small number of cells cycle at a roughly normal rate. A kinetic definition of accumulative and proliferative tumour growth is introduced. Tumour growth is termed accumulative if growth appears to result from a decreased relative cell loss rate rather than an increased relative cell production rate. According to this definition, the kinetics in CLL may be classified as accumulative. In absolute terms, however, the number of lymphoid cells produced per unit of time was found to be far higher in CLL than in the healthy state.