BACKGROUND Granulomatous myocarditis may develop into cardiomyopathy and severe congestive heart failure that requires implantation of a left ventricular assist device (LVAD). METHODS Left ventricular (LV) core samples were collected from 177 patients with severe heart failure at the time of LVAD implantation, and samples were histologically examined and graded for severity of hypertrophy and fibrosis. Granulomatous myocarditis incidentally seen in a subset of samples was characterized by staining and culturing for mycobacteria and fungi. Various clinical parameters in these patients were analyzed. RESULTS Of the 177 LV core samples examined, 6 (3.4%) showed nonnecrotizing granulomatous inflammation in the myocardial wall. Stains and cultures for mycobacteria and fungi were negative. All six patients [three women, three men; five African American, one Asian; mean age, 52±9 years (range, 41-61 years)] had arrhythmias and required an automatic implantable cardioverter defibrillator. Before LVAD implantation, the patients' mean cardiac index was 1.8±0.4 l/min/m(2); cardiac output, 2.9±0.6 l/min; and ejection fraction, 20±2%. One year after LVAD implantation, one patient had undergone heart transplantation. At 2 years, a second patient was transplanted, and one died. At 3 years, a third patient was transplanted and died postoperatively; two patients remained on support. No clinical evidence indicated involvement of other organs or recurrence in the transplanted patients. CONCLUSIONS The incidental diagnosis of granulomatous myocarditis in our patients indicates that histological study of LV core samples in patients who undergo LVAD implantation may contribute to the diagnosis and be a consideration in the management of the underlying cause of heart failure.