A 32-yr-old male diagnosed with myelodysplastic syndrome underwent an unmanipulated, unrelated, HLA matched, peripheral blood stem cell transplantation. The patient and donor were both blood type O, CcDEe. Twelve weeks post-transplantation, he developed acute autoimmune hemolytic anemia (AIHA). He was transfused multiple times with washed O red cells. High-dose steroid therapy was initiated and he underwent splenectomy; however, AIHA was refractory to therapy. The patient was further treated with combined treatment modalities including immunosuppressive therapy with mycophenolate mofetil and cyclosporine and three cycles of plasma exchange, and AIHA responded to treatment. This is the third case of AIHA complicating hematopoietic stem cell transplantation reported in Korea. Since AIHA is relatively common after hematopoietic stem cell transplantation, accurate and timely diagnosis of the disease and treatment strategies with multiple modalities are necessary.