We present the preliminary results of bone marrow transplantation for severe aplastic anaemia using the new immunosuppressive drug Cyclosporin A. We have compared the first seven patients treated with Cyclosporin A as post-graft immunosuppression with 14 similar patients with severe aplastic anaemia transplanted using methotrexate as the immunosuppressive drug in the post-graft period. All seven of our patients treated with Cyclosporin A have engrafted and so far there have been no rejection episodes, although the follow up period is short (greater than 94 to greater than 209 d). These results compare favourably with the methotrexate group of patients in whom five of the 15 patients either failed to engraft or subsequently rejected the graft. We have not encountered irreversible renal or hepatic failure due to Cyclosporin A and so far other effects such as hirsutism and mental changes have not been clinically significant. Two out of the seven patients treated with Cyclosporin A developed severe graft-versus-host disease (Grade III--IV). Four of the nine patients at risk in the methotrexate group developed severe (Grade III--IV) graft-versus-host disease.