35 patients were treated for acute myeloid leukaemia or acute lymphoblastic leukaemia with allogeneic bone-marrow grafts from a parent, child, or sibling who was mismatched at the major histocompatibility complex (MHC). 11 of these patients are alive at least 6 months after grafting, 5 of them after more than 2 years. Of the 15 patients aged under 20 at the time of the graft, 8 are alive and well 6 months to 3 years later. Cyclosporin A was given to all patients after grafting. 1 patient died of acute graft-versus-host disease and in 2 other cases this was a major factor in their death. Graft failure caused the death of 2 patients. 4 patients died of recurrent leukaemia. A fatal complication in 12 patients was pulmonary oedema, often associated with convulsions, intravascular haemolysis, and renal failure. Some of these patients had viral or bacterial infections, but in the majority the syndrome was not associated with demonstrable infection. This syndrome, in which the essential lesion appears to be vascular, was much more common in recipients of mismatched than matched grafts. 3 others died from lung disease in which infection was a factor.