For many years, the transplantation of hematopoietic stem cells has been used to treat some diseases of the hematopoietic system. For a very long time, only bone marrow was used as a source of hematopoietic stem cells for this method of treatment. However, to comply with allogeneic bone marrow transplantation, an antigenically compatible donor is necessary. Transplantations from unrelated donors are associated with increased risk of a graft-versus-host reaction, transplant rejection and, consequently, increased mortality. Many years ago, it was found that umbilical cord blood as well as bone marrow and peripheral blood contains hematopoietic stem cells and mesenchymal cells able to differentiate into different cell types and that the umbilical cord blood can be a source of stem cells for transplantation. Following this discovery, numerous attempts were made for its potential use in the treatment of hematologic diseases, metabolic diseases as well as regenerative medicine. Umbilical cord blood stem cells exhibit intermediate characteristics between embryonic and adult stem cells. They are distinguished from the latter by telomere length, telomerase activity, and lower risk of accumulation of DNA mutations or chromosomal aberrations. The only transplantation limitation appears to be the amount of cord blood collected, which on average is sufficient for transplantation in a 40-50 kg child. Collection of cord blood is a simple, short-lasting treatment, not causing any danger for a newborn or the mother. Umbilical cord blood is obtained during labor, and then frozen and stored at cord blood banks all over the world.