BACKGROUND Immunoglobulin (Ig)M warm autoantibodies (AABs) usually cause severe autoimmune hemolytic anemia (AIHA) and, in some cases, red blood cell (RBC)-bound IgM cannot be detected. We describe a simple dual antiglobulin test (DDAT) for diagnosing such cases. METHODS A patient with erroneously suspected cold agglutinin syndrome was investigated. The direct antiglobulin test (DAT) was performed using standard techniques and dual (two stages) antiglobulin reagents (IgG rabbit anti-human IgM, IgG goat anti-rabbit IgG). RESULTS A cold agglutinin syndrome was diagnosed initially, as the patient's serum was reactive with RBCs at a temperature of 28 degrees C or less, and the DAT was strongly positive with anti-C(3)d. Six months later, the patient was reexamined at this hospital due to progressive hemolysis. His RBCs were found to be coated with IgM warm AABs that only became detectable using a DDAT, and his serum contained only a weak cold agglutinin. The hemolysis remained refractory to treatment with prednisolone and also prednisolone plus azathioprine, but gradually improved after treatment with prednisolone plus cyclophosphamide. CONCLUSIONS Weak or nonagglutinating RBC-bound IgM warm antibodies can be identified by the presented DDAT.