The electromicroscopic observation of atrialbiopsy was carried out in 10 patients with ventric-ular septal defect (VSD) and middle, severe pul-monary hypertension (PH). The observation sug-gests that there are myofibrillar lysis to disappearwith various degrees, dilated rough endoplasmreticulum and the increase in lipofuscin granulesand lipid droplets; mitochondral hyperplasia, theirsize and shape being different, disorganizing of mi-tochondria, disintegration of cristae, mitochondralretrograde degenration and decreased matrix elec-tron density. The dilated intercalated discs and in-crease in myocardial fibrous tissue were found inpart of patients. These structural changes areprobably due to chronic myocardial ischemia.