An 89-year-old woman presented to our hospital with hemolytic anemia and a high titer of cold agglutinins. Red cell agglutination was observed on a blood smear. Agglutination visibly decreased after warming the blood; therefore, the patient was diagnosed with cold agglutinin disease (CAD). Bone marrow aspiration revealed no infiltration of malignant cells. Computed tomography indicated moderate splenomegaly. The patient had neither an infection nor autoimmune disease. Initial steroid therapy was ineffective and hemolysis worsened. Meanwhile, thrombocytopenia, delirium, fever, and schistocytes in the blood were observed. The progression of hemolysis was attributed not only to CAD but also to coexisting thrombotic thrombocytopenic purpura (TTP) because of the decreased ADAMTS 13 level. Autopsy revealed mild paraaortic lymphadenopathy and splenomegaly. Microscopic examination revealed lymphoma cell infiltration in the spleen, liver, bone marrow, and paraaortic lymph nodes. These observations suggested that TTP and CAD were both secondary complications. This case highlights the importance of an autopsy for the detection of latent lymphoma, which can be difficult to diagnose before the patient's death. Careful examination to exclude lymphomas is important in patients with CAD at the time of diagnosis.