Background: Donor mixed chimerism (MC) is an increasing problem after hematopoietic stem cell transplantation (HSCT) for nonmalignant diseases. Procedure: In this study, a self-administered questionnaire was used to retrospectively compare efficacy and safety in 49 patients undergoing second HSCT (n = 13) or donor lymphocyte infusion (DLI
n = 36) as treatment for MC. Results: The response rate to DLI of patients with secondary graft failure (GF) (25.0%) was significantly lower than that of patients without secondary GF (81.3%
P = 0.041). Among patients undergoing DLI, the rates of successful response were significantly higher in patients having at least 30% donor chimerism (94.1%) than in patients having less than 30% donor chimerism (61.1%
P = 0.041). Furthermore, the rates of successful response were significantly higher in patients receiving larger first or maximum doses of DLI. Sixteen (50.0%) of 32 patients without secondary GF attained complete chimerism after DLI. The cumulative incidence of grade II–IV acute graft-versus-host disease and cytopenia was 37.6 and 26.1%, respectively. Conclusions: DLI yields promising response rates in most patients with higher donor chimerism levels, whereas second HSCT is more likely to benefit patients with lower donor chimerism levels.