The distribution of gross myocardial lesions and the relative density of the inflammatory cells in various myocardial locations (subepicardial and subendocardial halves of the left ventricular free wall, right ventricular and left ventricular halves of the ventricular septum, and the right ventricular free wall) were studied in 14 patients (11 male [79%], 8 black [57%], aged 7 months to 65 years [median 16 years]) who died suddenly of acute mononuclear-cell myocarditis. Grossly visible myocardial lesions were present in 8 patients, and preferentially involved the subepicardial regions of the left ventricular free wall. The highest mean number of mononuclear cells were seen in the subepicardial regions of the left ventricular free wall (193 cells per high-power field) and the lowest in the right ventricular one half of the ventricular septum (92 cells per high-power field, p < 0.001). Thus the predominant location of the gross myocardial lesions in acute mononuclear-cell myocarditis is the subepicardial region of the left ventricular free wall. A relatively low density of mononuclear cells are found in the right ventricular half of the ventricular septum, the main location of endomyocardial biopsies.