The limited life-span of most blood cells requires continuous production of cells which in adults may exceed 1012 cells/day. This impressive production of cells (approximately 4.1015 cells over a life time) is achieved by the proliferation and differentiation of committed progenitor cells which themselves are derived from a population of pluripotent stem cells with self-renewal potential. In adults, the large majority of stem cells are found in the bone marrow among cells with a CD34 + CD38- phenotype. Interestingly, small but significant numbers of such cells can be found in the circulation. The frequency of circulating CD34 + CD38- cells can be dramatically increased by treatment with certain compounds including cytokines. Such "mobilized" peripheral blood stem cells have become an important alternative to bone marrow in stem cell transplantation procedures primarily because engraftment is more rapid. The latter is almost certainly related to the increased numbers of primitive CD34 + CD38- cells capable of engrafting the bone marrow in blood versus bone marrow stem cell grafts [1]. Paradoxically, the large majority of "candidate" stem cells in adult bone marrow are quiescent cells. One possibility is that stem cells, like other somatic cells, have only a limited replicative potential (< 100 divisions). This hypothesis is supported by two key observations and the consideration that, in theory, 52 divisions can yield 4.1015 cells. First, it was shown that "candidate" stem cells purified from fetal and adult tissue display marked functional differences in turn-over time and the ability to produce cells with stem cell properties [2]. Secondly, these functional differences were found to correlate with a measurable loss of telomere repeats [3], despite the presence of low but readily detectable levels of telomerase in all purified cell fractions [4,5]. In order to address questions about the role of telomeres in normal and malignant hematopoiesis, we developed quantitative fluorescence in situ hybridization [6]. With this technique the length of telomere repeats at individual chromosome ends can be reliably estimated using optical density measurements from digital images of metaphase chromosomes after fluorescence in situ hybridization with directly labeled (CCCTAA)3--Peptide Nucleic Acid Probe [6,7]. Furthermore, we recently showed that this method can be adapted to measure the total telomere repeat content of cells by flow cytometry [8]. Here some issues in studies of hematopoietic stem cells are discussed in relation to rapidly accumulating information about telomere biology.