A patient who developed recurrent leukemia more than six years after marrow grafting from an HLA-identical same-sex sibling is reported. Difference in DNA restriction fragment length polymorphisms between donor and host demonstrated that the DNA in the recurrent leukemia sample was probably of donor origin. Possible mechanisms that could explain the long latent period between transplantation and expression of leukemic transformation are discussed. We conclude that future cases of late leukemic recurrence after marrow grafting should be studied to determine whether, in contrast to early relapses, late relapses occur in donor cells in most or all instances.