Warm IgM autoantibodies occur in association with IgG-class and/or IgA-class immunoglobulins in approximately 30 percent of patients with warm-type autoimmune hemolysis. They may be classified as agglutinins or hemolysins, which may be incomplete or complete, depending on in vitro serology; they almost always bind complement. Autoimmune hemolytic anemia solely due to warm IgM autoantibodies is exceedingly rare. We report two cases of the incomplete agglutinin type. The autoantibodies were confirmed as IgM by their ability to rebind to normal red blood cells (RBCs) after elution; the absence of small increases in RBC-bound IgG and IgA was shown by a sensitive enzyme-linked antiglobulin test. Patient 1 was a 64-year-old female with non-Hodgkin's lymphoma, with a hemoglobin of 50 g/L and haptoglobin of < 0.1 g/L. Direct antiglobulin tests were positive for IgM, C3d, and C3c; only IgM was present in an eluate. The serum contained a weak autoantibody at 37 degrees C and tests for hemolysins were negative. The patient suffered chronic hemolysis and required intensive treatment, including splenectomy. Patient 2 was a 65-year-old female; the hemoglobin was 78 g/L and the haptoglobin was < 0.1 g/L. Direct antiglobulin tests were positive for IgM and C3d; an eluate contained only IgM. No free autoantibody was present in the serum and tests for hemolysins were negative. Two serious infections occurred and the hemolysis remained chronic, requiring continuous treatment during the 4 months she was followed.