Human umbilical cord blood (CB) represents a unique source of transplantable hematopoietic progenitor cells. Potential advantages of using CB relate to the high number and quality of hematopoietic stem and progenitor cells present in the circulation at birth and to the relative immune immaturity of the newborn immune cells. Discussed in this review are: (a) Quantity and quality of immature hematopoietic stem and progenitor cells from cord blood; (b) Immune cells in cord blood including the number of B- and T-lymphocytes, as well as natural killer cells and characterization of their functional capacities; (c) The need of an international CB transplantation registry and the availability of cord blood banks. Although still in its infancy, human CB progenitor cells hold considerable potential for in vitro expansion and to transplant the adult recipients. In addition, the CB repopulating progenitor cells can serve as targets for gene transfer and long-term treatment of genetic inherited diseases, cancer and some immunodeficiencies.