Of 144 patients with a positive direct antiglobulin test and having autoimmune hemolytic anemia (AIHA), 12 (8.3%) satisfied diagnostic criteria for both warm antibody AIHA and cold agglutinin syndrome. All 12 patients had IgG and C3d sensitizing their erythrocytes, and samples of their serum contained IgM cold autohemagglutinins optimally reactive at 4 degrees C, but with a high thermal amplitude to 37 degrees C, and IgG warm autoantibodies. All red blood cell eluates contained IgG warm autoantibodies. The 12 patients had severe hemolytic anemia that responded dramatically to corticosteroid therapy, with the mean hemoglobin level increasing from 6.3 to 12.9 g/dL. Five patients (42%) had systemic lupus erythematosus, one patient (8%) had a non-Hodgkin's lymphoma, and six patients (50%) had idiopathic AIHA; four patients (33%) had concomitant thrombocytopenia (Evans' syndrome). Nine patients (75%) were female. Four patients had unexpected alloantibodies potentially capable of in vivo hemolysis of transfused blood. Because of the severe hemolysis, the serologic findings, and the dramatic initial clinical response to corticosteroid therapy, these patients represent a distinct catagory of AIHA and should be given corticosteroid therapy quickly.