Hemopoietic cell proliferation was studied in a patient suffering from preleukemia characterized by peripheral pancytopenia and hypercellular bone marrow with ineffective erythropoiesis. Two years later when overt acute myelogenous leukemia had developed the study was repeated. The kinetics of proliferation were investigated by a new method which allows evaluation of the rate and time of DNA synthesis in individual morphologically defined cells. Erythropoiesis was found ineffective to the same degree in both stages of disease. The rate of erythroid cell proliferation, however, was reduced in overt leukemia only. The myeloid system showed a grossly reduced production rate of myeloblasts in preleukemia whilst the same parameter was strongly increased in leukemia. This high production rate of myeloblasts in overt leukemia was interpreted as indication of a far-reaching self-maintenance of the myeloblast pool in this stage of disease. The proliferative activity of the individual myeloblasts was reduced already in preleukemia, and even more so in leukemia. In order to explain the amplification of the myeloblast pool with the onset of overt leukemia a change in the mode of myeloblast divisions is assumed. For this a transition from steady state to some degree of exponential growth gives the most plausible explanation.