39 clinical bone marrow transplants (BMT) for leukemia are described. In a historical control series of 18 patients in whom BMT was performed after all chemotherapeutic resources had been exhausted, there is only 1 long-term survivor (5.5%), 8 patients died from GvH reaction, 6 from interstitial pneumonia and 3 from recurrent leukemia. Since 1979 an attempt has been made to transplant patients under optimal conditions (1st complete remission) and cyclosporin-A (CyA) has been used for prophylaxis of GvH reaction instead of MTX. 11 patients were transplanted according to our original proposal (AML and ALL in first remission, CML in chronic phase): 10 have survived without evidence of leukemia (91%), 1 AML died in relapse. 10 patients were grafted in second or later remissions or early relapse: 5 have leukemia-free survival (50%), 1 is living with a relapse. In this group 3 deaths were due to recurrent leukemia and 1 to CMV-infection. In our experience BMT under optimal circumstances does not involve a risk of early mortality and the chances of recurrent leukemia are reduced. Severe or chronic GvH reaction is not seen under CyA. BMT is the treatment of choice for patients with histocompatible sibling donors.