Erythroblasts (EBL) are normally not observed in peripheral blood, but may be found in patients suffering from a variety of severe diseases. The detection of EBL in peripheral blood has been shown to be associated with a poor prognosis. However, the clinical significance of peripheral erythroblastosis after hematopoietic stem cell transplantation (HSCT) has not been evaluated. We retrospectively analyzed the records of 161 patients who underwent HSCT at our hospital from June 1995 to October 2001. EBL at any level were detected in 94% of the patients. Forty-four and 11 patients experienced erythroblastosis exceeding 200 and 1,000/ul, respectively. The erythroblast count was higher in patients who died than in the survivors (geometric mean value 184 vs. 100/ul, P=0.01). High-level erythroblastosis ( >1,000/ul) within 180 days after HSCT was associated with an extremely poor prognosis (median survival 22.5 days). Among the possible confounding factors, the use of total body irradiation (RR 2.35, 95% CI 1.22 - 4.54, P=0.011) and the disease status before transplantation (RR 2.51, 95% CI 1.15 - 5.49, P=0.021) were independent significant factors for erythroblastosis after HSCT. As for post-transplant events, a high EBL concentration was frequently preceded by graft-vs.-host disease, thrombotic microangiopathy, hypoxia, and hematological relapse.