- Few cases are found in the literature regarding autoimmune hemolytic anemia which is Coombs' test positive in kidney transplant patients, although hemolytic uremic syndrome due to cyclosporin and FK506 has been well described. In the following, we describe a case of severe life-threatening Coombs' test negative autoimmune hemolytic anemia after kidney transplantation. Soon after undergoing renal transplantation, the patient presented with hemolytic anemia. Kidney biopsy, routine Coombs' test, gel filtration and flow-cytometric assay were undertaken. Kidney biopsy ruled out hemolytic uremic syndrome; although Coombs' test and gel filtration assay were negative, flow cytometry revealed circulating antierythrocytic autoantibodies. Our findings indicate that flow cytometry may be an efficient method in the diagnosis of hemolysis of unknown origin in transplant patients. We further hypothesize that the underlying mechanism of autoimmune hemolytic anemia is related to the passenger B lymphocytes in the graft.