A case of leukemia relapse in a 15-year-old girl occurring 5 years after successful allogeneic bone marrow transplantation for acute non-lymphocytic leukemia is reported. Laboratory findings demonstrated that this relapse was in host cells and had the same phenotype as the original leukemia. Incomplete lymphoid chimerism after bone marrow transplantation might have resulted in an inappropriate graft-versus-leukemia effect.