Four patients with Diamond-Blackfan syndrome (congenital hypoplastic anaemia) whose disease was resistant to corticosteroid treatment and who were red blood cell transfusion-dependent, were given marrow grafts from allogeneic human-leucocyte-antigen (HLA)-identical siblings. The patients were conditioned with regimens including cyclophosphamide and busulfan. Three of four patients had sustained and complete marrow engraftment. One patient showed early signs of haematopoietic recovery but died on day 35 of pulmonary toxicity. The three surviving patients are well with normal haematopoiesis and Karnofsky performance scores of 100%, 3.0, 7.4 and 10.6 years after transplantation. Congenital hypoplastic anaemia can be treated successfully by allogeneic marrow grafts.