One hundred and nineteen patients with leukaemia have received CSA as prophylaxis against acute GVHD following sibling MHC matched (84) or family member mismatched bone marrow transplants (35). four recipients of matched transplants (3%) died of acute GVHD, a marked improvement on our previous results using methotrexate (26 patients; 27% died of GVHD). Thirty-five patients received mismatched transplants, 15 were under the age of 20 years and eight are alive and in remission, the longest survivor being one at 2 1/2 years after transplant.