The increasing interest in geriatric medicine demands an understanding of aging in human tissues. The changes in the human heart are an important aspect of this understanding because cardiovascular diseases are a leading medical problem in the elderly. The published data about age-related changes in the human myocardium remain incomplete and occasionally are controversial. An ongoing study of the hearts of people aged 80 years and older is being conducted at our institution. Various aspects of morphologic changes in these hearts were studied. In this report, we discuss the presence and extent of myocardial fibrosis, namely, the interstitial type. Two types of fibrosis are recognized: (1) scarring, the fibrotic replacement of lost myocardium, usually of vascular origin; and (2) interstitial, in which a delicate fibrotic net encircles single myocardial fibers. The morphologic findings were correlated with the following clinical findings: hypertension, congestive heart failure, emphysema, cor pulmonale, and coronary artery disease. The results support the hypothesis that interstitial fibrosis develops independently of the above-mentioned clinical conditions and may be considered as a true aging process.