The pathological findings and available clinical data in 15 necropsy cases of HOCM, aged over 61 years, are reported. Three patients were in the eighth decade and 4 in the ninth; 8 were women. Five presented as sudden death, 2 died in congestive cardiac failure, and 7 died of unrelated conditions and HOCM was an apparently incidental postmortem finding. Compared with cases under 60 years, the hearts of the elderly patients were heavier and less likely to show typical asymmetrical hypertrophy, the free wall of the left ventricle also being thickened in two-thirds of the cases over 60 years. Most of the elderly cases showed a distinctive band of fibrous thickening over the upper part of the interventricular septum. This lesion had a "mirror image" relation to the lower part of the aortic surface of the anterior mitral cusp, with the histological features of a friction lesion. It appears to be a morphological expression of the systolic contact of anterior mitral cusp and interventricular septum seen on cineangiography and thus diagnostic of HOCM. Once formed, the fibrous band appears to persist even if the obstructive element disappears. It is, therefore, a valuable diagnostic feature indicating a diagnosis of HOCM in an age group where the morphology is usually not the classical asymmetrical form and in which this diagnosis is usually not considered.