Cytokine-mobilized peripheral blood stem cell products are increasingly used for hematopoietic reconstitution after myeloablative therapy. Favorable engraftment kinetics, the ease of harvest, and the large number of CD34+ cells obtained that allow for graft manipulations (ie, tumor cell or T-cell depletion) have made this stem cell source an attractive alternative to marrow. More recent data suggest that in addition to the increased number of CD34 cells, there may be also qualitative differences between leukapheresis products and marrow. In the allogeneic transplantation setting, the one log more T cells contained in granulocyte colony-stimulating factor-mobilized peripheral blood mononuclear cells compared with marrow has not translated into more severe graft-versus-host disease, indicating possible differences in T-cell or accessory-cell function. Whether such differences will compromise graft-versus-leukemia effects and disease-free survival remains to be seen. Nevertheless, it is reasonable to speculate that cytokine-mobilized peripheral blood products may eventually replace marrow as a source for hematopoietic stem cells. However, each new mobilization strategy needs to be evaluated carefully, as comparable increases in CD34 cell numbers may not necessarily affect the same, as yet underlined, qualitative changes that make this product so attractive.