Naturally occurring hypertrophic cardiomyopathy (HCM) in pigs in generally characterized by unexplained cardiac hypertrophy with abnormal histological features (Liu et al., 1994; Dai et al., 1995). The histological alterations in HCM-affected hearts are characteristic, and can be used to diagnose the disease (Dai et al., 1995). Briefly, these are marked disorientation of cardiac muscle cells, thickening of the intramural coronary arterial wall with a narrowing of the lumen, endocardial and myocardial fibrosis. A high incidence of HCM in a population of pigs strongly suggests a hereditary basis and the Pig Research Institute, Taiwan has, therefore, endeavoured to produce a specific strain of HCM pigs. The purpose of the present study was to determine the ultrastructural changes occurring in pigs with naturally occurring HCM.