Endomyocardial biopsies from 97 normal donor hearts were examined. Morphometric analysis showed: mean myocyte diameter 22.21 +/- 6.93 mu, mean nuclear dimension 7.32 +/- 2.33 mu, mean nuclear/sarcoplasmic ratio 0.33 +/- 0.02. 31 biopsies showed enlarged myocytes (mean diameter 31.65 +/- 3.98 mu) with increased nuclear size (mean 10.45 +/- 1.39 mu), but preserved nuclear/sarcoplasmic ratio (mean 0.33 +/- 0.01). The mean age of these latter subjects was significantly higher. Endocardial thickness mean value was 17.73 +/- 4.58 mu, but in 28 cases the value exceeded the considered upper normal limit of 20 mu. Interstitial mononuclear cells were rare and randomly present. Interstitial fibrosis was observed in 15% and focal fibrosis in 27% of cases. Our results show that histology of biopsies from clinically normal hearts can widely vary, sometime overriding the pathologic boundaries. These apparently "benign" abnormalities should be kept in mind when specific pathologic substrates of cardiac diseases have to be defined.