In haemopoietic stem-cell transplantation (HSCT), donor engraftment follows ablation of the recipient’s marrow and immune system by conditioning chemo radiotherapy (before the transplantation) and by the alloimmune action (graft-versus-host marrow) of the engrafted donor cells against residual cells in the recipient. These processes are not always easy to control, and can lead to graftversus-host disease (GVHD) when non-haemopoietic cells (eg, gut, skin, liver, and lung) are targeted. This outcome generates much risk of morbidity and mortality for the recipient. Therefore risk of transplantation is weighed against risk of disease in decision algorithms. The difficulty is that although HSCT might be the most secure way to be alive in three decades, it is also the most likely way of being dead in 3 months. HSCT is indicated for haematological malignant disease in which conditioning therapy and the graft-versustumour effect are instrumental in sustaining disease remission. Transplantation successfully eradicates nonmalignant diseases of the blood, including common disorders such as sickle-cell anaemia and thalassaemia, and some inborn errors of metabolism for which donor cells act as a permanent source of deficient enzyme. 1 Conditioning therapy or uncontrolled graft-versus-host marrow reactions resulting in clinical GVHD have no benefit in such disorders. Additionally, conditioning drugs