An autoaggression graft-versus-host (GVHD)-like syndrome or engraftment syndrome (ES) presenting with skin rash, fever, and other clinical findings can accompany the early phase of engraftment after autologous peripheral blood stem cell (PBSC)/bone marrow (BM) transplantation. Because ES was suggested to be analogous to GVHD, we have investigated whether ES was associated with any graft-versus-tumor effect that would affect disease progression and survival in breast cancer patients. Eighty-five consecutive patients who received BM/PBSC transplantation for breast cancer (stages II-IV) between July 1991 and July 1997 with minimum 2-year follow-up were studied. Median follow-up time was 892 days (range, 106-2913 days). Thirty-three patients (39%) developed ES. The incidence of relapse/progressive disease for the whole cohort was 61% and was similar in patients who developed ES compared with those who did not. However, there was an increased rate of mortality observed among the patients who had developed ES versus those who had not, although it was statistically not significant, (52% versus 31%, respectively; log rank, P = .08). Increased mortality rates due to disease progression were seen in all patients with ES regardless of their disease stage. In relapsed patients, median survival time after transplantation was 586 days for those with ES versus 847 days for those without ES, and the mortality rate was 85% (17/20) versus 51% (16/31) (P = .008) for those with or without ES, respectively. Visceral (lung, liver, brain, adrenal) or multiple-site relapses were observed in 85% of patients with ES versus 52% without ES (P = .01). In conclusion, whereas there was no effect of ES on relapse rate, a surprisingly significant increase in disease-related mortality rates among relapsed breast cancer patients with ES was found. Thus, patients with ES should be considered for close follow-up and further therapy posttransplantation.