Giant cell myocarditis (GCM) is a rare condition whose histologic hallmark, the multinucleate giant cell, is of debated origin (monocytic v myogenic). We report the case of a 46-year-old woman with a previous diagnosis of ulcerative colitis who rapidly deteriorated and died as the result of refractory ventricular tachyarrhythmias. Postmortem examination showed a diffuse infiltration of the myocardium by round cells and multinucleate giant cells. Immunohistochemically, round cells were demonstrated to be T lymphocytes admixed with monocytes. Multinucleate giant cells expressed monocytic markers (MAC 387, lysozyme) and were negative for muscle markers (actin, desmin, myoglobin). This case illustrates the monocytic and macrophagic nature of multinucleate giant cells and lends support to the autoimmune hypothesis of GCM by the concurrence of the latter with ulcerative colitis.