Although sudden cardiac death is often reported in epileptic or psychiatric patients, the etiologyis not fully understood. The aim of this study was to examine whether morphological changes of the heart contribute to sudden cardiac death in epileptic or psychiatric patients, compared with control decedents.Cases of sudden cardiac deathsdue to fatal arrhythmia in patients with epilepsy or psychiatric disease were selected from forensic autopsies performed at the Shiga Prefecture from 2014 to 2021.As the control group, autopsied healthy decedents were selected. The heart weight to normal heart weight ratio, ventricular wall thickness, cardiomyocyte width, nuclear diameter and nuclear density of the left and right ventricle and interventricular septum were examined. The left and right ventricular wall thickness and cardiomyocyte to be smaller in epileptic patients, however, no significant differences were found. The nuclear diameter/body surface area of the left ventricle was significantly smaller in epileptic and psychiatric patients than in control patients.The smaller nuclei of cardiomyocytes in epileptic and psychiatric patients may relate to the etiology of sudden cardiac death.