Despite a slight decrease in mortality over the last decade, breast cancer still remains a leading cause of cancer-related death in women. Although anti-tumor effects have been observed after allogeneic stem cell transplantation (SCT), this treatment is not standard care owing to graft-versus-host disease (GVHD) and scarcity of suitable donors. With the aim of reducing treatment-related mortality and increasing donor availability in clinical situations, we developed a preclinical mouse model that combines nonmyeloablative conditioning with the use of haploidentical donor-recipient pairs. To mimic active disease, CB6F1 mice were inoculated with 5 x 10(4) 4T1 mammary carcinoma cells 10 days before transplantation. Keratinocyte growth factor (KGF) was used as GVHD prophylaxis. Syngeneic (CB6F1) SCT did not cure any of the mice and KGF treatment did not influence tumor development. After transplantation with haploidentical (B6CBAF1) bone marrow and splenocytes, however, tumor outgrowth was reduced and long-term disease-free survival (>3 months) was observed in 9/18 (50%) (P=0.0011) of the animals. We conclude that in a model of murine breast cancer, a graft-versus-tumor effect can be induced by a nonmyeloablative haploidentical SCT procedure.