Hematopoietic stem cell (HSC) enumeration is crucial to predict the engraftment potential of a given HSC collection, and currently involves the surrogate count of nucleated cells, CFU or CD34+ cells. However, there is raising evidence that CFU are HSC involved in short-term but not in long-term reconstitution, and that only a small fraction of all CD34+ cells have long term multilineage engraftment potential. In this regard, there is evidence that cord blood (CB), bone marrow (BM) and peripheral blood (PB) derived HSC are highly heterogeneous for a number of antigens useful for HSC enumeration by flow cytometry. Moreover, there is a raising evidence that a CD34 human HSC might exist. The CD34 HSC has been already described in animals and in human Hoechst 33342 negative HSC. This notwithstanding, clinical data have clearly demonstrated that purified allogeneic CD34+ cells can reconstitute the myeloid and the lymphoid lineages in myeloablated recipients. In the lack of a suitable marker for CD34 HSC enumeration, it is hard to predict the role of CD34 HSC in hematopoietic reconstitution after transplantation. On the other hand, these cells might be a better target for HSC expansion and gene transfer.