Characteristically, the RV in ARVC is replaced with a fibrofatty tissue. Morphologic alterations of ARVC usually begin in the subepicardium or mediomural layers (8,18) of the RV and progress to the endocardium with fibrofatty replacement of myocytes and thinning of the wall. The regions of RV most frequently involved are the RV inflow area, the apex and the infundibulum. These three areas form “the triangle of dysplasia” (2,19). However, small amounts of fat are present in the epicardial layer and within the RV myocardium in normal subjects. Fontaine et al. (20) examined the hearts at necropsy in 140 individuals with no history of heart disorders. Over 50% of the subjects had fat within their RV myocardial fibers, and the presence of intramyocardial fat increased with age. Therefore, histologic diagnosis of ARVC may be difficult in borderline cases. To avoid overdiagnosing ARVC, Angeline et al. (21) have proposed that the presence in biopsy sections of more than 3% of fibrous tissue and more than 40% of fatty tissue is highly suggestive of ARVC. These authors also emphasized the importance of identifying coexisting myocardial fibrosis in making the diagnosis. In a forensic autopsy study of 20 patients with ARVC who died suddenly (22), the fatty replacement involved the outer half of RV free wall in 27%, the outer two-thirds in 28% and the entire wall thickness in 45% of the cases. Interestingly, the endocardial muscular trabeculae are generally spared but may occasionally also be atrophied (22- 23). The LV was involved in 40% of cases in this report (22), although other reports have identified LV involvement in up to 76% of individuals with ARVC examined at necropsy (6). When the LV is involved, the fibrofatty replacement can affect both the septum and LV free wall, either diffusely or, more often, regionally with a predilection for the posteroseptal and posterolateral areas. In the LV, fatty replacement of the myocardium has a predilection for the subepicardial and midventricular wall (6).