Hematologic cancers that recur after allogeneic hematopoietic stem-cell transplantation are often difficult to treat. A small pilot study suggests that ipilimumab may induce durable responses in a subgroup of patients with these cancers. Allogeneic hematopoietic stem-cell transplantation (HSCT) is the only cure for many patients who have advanced hematologic cancers, principally through the induction of a graft-versus-tumor effect. 1 Unfortunately, more than one third of patients who have undergone transplantation have a relapse of disease. 2 The prognosis for these patients is poor; the majority die within 1 year after relapse despite salvage chemotherapy, donor-lymphocyte infusion, or retransplantation. 3 – 5 Immune escape (i.e., tumor evasion of the donor immune system) contributes to relapse after allogeneic HSCT, and immune checkpoint inhibitory pathways probably play an important role. 6 The engagement of cytotoxic T-lymphocyte–associated protein 4 (CTLA-4) and programmed . . .