A 73-year-old man presented with dyspnea and atrial flutter associated with an amyloid tumor in the heart. IgM-κ gammopathy, hypercalcemia, and extensive cardiac and mediastinal invasion suggested a malignant lymphoid or plasma cell process. Although amyloidoma is generally considered to be a benign tumor, the aggressive features of this case mandated chemotherapy because the critical location rendered the tumor inoperable. This case provides noteworthy evidence in support of a possible pathogenic relationship between amyloidoma and plasmacytoma by virtue of dual representative features: localized amyloid infiltrated with plasma cells and the associated gammopathy. Local and systemic malignant features lend additional support to this hypothesis.