Examples of patients are used as a basis for discussing the treatment of severe aplastic anaemia with clonal chromosomal aberrations in haematopoietic cells, the aplastic form of paroxysmal nocturnal haemoglobinuria and myelodysplasia, with allogeneic bone marrow transplantation. Transplantation with marrow from an HLA-identical sibling donor has curative potential in selected patients with these disorders. The procedure should preferably be carried out before the patients have received massive treatment for the complications of marrow failure. The use of matched unrelated bone marrow donors is still at the investigation stage.