Umbilical cord blood has become a valuable alternative source of haematopoietic stem cells for allogeneic transplantation. However, largely because of the impact of cell dose on engraftment, risk of transplant-related mortality and survival, progress in the field has been largely restricted to children. Nevertheless, substantial clinical experience with umbilical cord blood transplantation clearly establishes its safety and effectiveness in young patients with a variety of malignant and non-malignant diseases. Although less well established in adults, new strategies are now being explored to address the obstacle of cell dose. This article reviews the state of the art and future directions with umbilical cord blood as a source of haematopoietic stem cells for transplantation in both children and adults.