In order to clarify the importance of the conduction system involvement in cases of sudden cardiac death, 35 autopsied hearts, obtained from the patients who died within one hour after the onset of a critical attack, were examined histopathologically and compared with 27 both age- and disease-matched and 30 only age-matched control hearts from individuals who had not died suddenly. The conduction system was serially sectioned using Lev's method and observed under a light microscope. Purkinje cell lesions, which made a structural maze compatible to the electrophysiological re-entry mechanism, abnormalities of the AV node artery and hypertrophy of the AV node and the bundle of His were more prominent in the sudden-death group with long QT syndrome, isolated ventricular fibrillation and AV and bundle branch blocks due to both ischemic heart and myocardial diseases. Sinus node fibrosis with minor anomalies of the sinus node artery was specifically seen in the sudden death of apparently healthy young males (Pokkuri disease). The same lesion, but with sclerosis of the sinus node artery, was seen in the cases of sudden death with hypertensive heart disease.