A morphological study was designed to assess on histological grounds the frequency and degree of interstitial adipose tissue in the myocardium of diseased and transplanted hearts. A review was conducted on 144 right ventricular histological sections (RVs) from hearts surgically resected for heart transplantation, 115 endomyocardial biopsies (EMB) from 100 patients investigated for dilated cardiomyopathy (DCM), and 309 biopsies from 26 heart-transplant patients. Interstitial lipomatosis (IL) was the preferred term to describe the mere presence of adipose tissue in the myocardium, and the arbitrary classification of grade 0, 1, and 2 IL was devised to collect data. Only grade 2 IL was considered as potentially pathological. Morphometric analysis was adopted on the 144 RVs from surgical hearts; only a morphological approach was preferred for the EMB. Grade 2 IL was observed in 58/144 RVs (40.27%), in 5/365 (1.3%) myocardial samples from 115 EMB in patients with DCM, and in 5/1296 (0.38%) myocardial samples from 309 biopsies of 26 transplants. Morphometric analysis of the surgical hearts revealed a case in which a significant grade 2 IL of the RV was associated with life-threatening ventricular arrhythmias.