Haplo-identical transplants may provide a possible benefit. They may enable an attack on cancer to occur. A graft vs. tumor or graft vs. leukemia effect occurs when a donor's mature immune cells come along with the stem cells in the transplant, and these immune cells recognize and attack as "foreign" the cancer cells found in the patient's body. This phenomenon helps prevent relapse and, in some cancers, can even be curative. To boost this graft vs. tumor effect, patients are sometimes given an infusion of a donor's haplo-identical immune cells (white blood cells) along with the stem cell transplant.

A risk with haplo-identical transplants is that of enabling an attack on the patient's tissues. This can happen when a donor's mature immune cells, called T cells, contaminate the stem cell graft. To minimize this risk, researchers sometimes deplete all T cells from the transplant, leaving the graft with stem cells only. While this can minimize the chance of graft vs. host disease, it also reduces the transplant's ability to attack any loitering cancer cells. An alternative option is for the doctor to prescribe steroid medications for the patient pre- and post-transplant. This helps to prevent graft vs. host disease, yet leaves T cells available for graft vs. tumor action.