Sudden unexpected deaths in young persons with noncontributory histories, autopsy results, and drug screen results are a common problem in forensic pathology. As part of the evaluation of such cases, the cardiac conduction system (CCS) should be studied. To determine the type and incidence of lethal CCS lesions, the authors reviewed their files of sudden unexpected cardiac deaths with particular attention to cases with causes of death in the conduction system. Cases of sudden cardiac death in patients aged < or=40 years during a 10-year period (Michigan) and a 4 year-period (Spain) were selected from the files. From this group, cases were identified in which the cause of death was a lethal change in the CCS. The portions of the heart containing the CCS were excised, and at least one hematoxylin and eosin slide and at least one trichrome or elastic trichrome slide per block were studied. In the two centers, 381 cases of sudden cardiac death were identified. The most common causes of sudden cardiac death were arteriosclerotic narrowing of the coronary arteries, cardiomyopathy, and myocarditis. In 82 cases, there was no identifiable cause of death even after complete gross and microscopic autopsy was performed, a medical history was obtained, and a drug screen was performed. In 11 cases, the CCS contained lesions that were considered lethal: narrowing of the atrioventricular node artery by fibromuscular hyperplasia (7 cases) and atrioventricular node tumors (4 cases). The 11 cases accounted for 2.9% of the 381 cases of sudden cardiac death and 11.8% of the indeterminable cases. It was concluded that examination of the CCS in deaths in which the gross and microscopic autopsy, history, and drug screen fail to provide a cause of death can yield a cause of death in a significant percentage of cases. If heart block was not documented during life and no explanatory lesions were found during routine cardiac examination, examination of the CCS can yield valuable information.