Objective: The aim of the present study was to assess the prevalence of subtle morphologic substrates, clinically unrecognizable, underlying sudden cardiac death (SCD) in young people with apparently normal heart. Methods: In the time interval 1979–1998, 273 consecutive cases of SCD in young people (≤35 years) which occurred in the Veneto Region of Italy were prospectively studied. Following exclusion of extracardiac causes of sudden death, the heart was examined according to a detailed morphologic protocol consisting of macroscopic and histologic examination, including study of the specialized conduction system by serial sections. Results: At macroscopic examination, 197 SCD victims (72%) had an overt underlying structural heart disease such as cardiomyopathy in 56, obstructive coronary atherosclerosis in 54, valve disease in 32, non-atherosclerotic coronary artery disease in 28, aortic rupture in 13, postoperative congenital heart disease in five, and other disease in nine. The remaining 76 cases (28%) (50 males and 26 females, aged 4–35 years, mean 23±5 years) had a macroscopically normal heart. A total of 28 of them (37%) had experienced one or more of the following prodroma: syncope, palpitations or both in 20, ECG abnormalities in 18 and arrhythmias in ten. In 79% of them, histologic examination disclosed concealed pathologic substrates consisting of focal myocarditis in 27 cases, regional arrhythmogenic right ventricular cardiomyopathy, mostly localized to RV outflow tract, in nine, and conduction system abnormalities in 24 (leading to ventricular preexcitation in 18 and heart block in six). In 16 hearts (6%) there was no evidence of structural heart disease even after histologic study. Conclusion: Macroscopic heart features were normal in nearly one-third of young SCD victims. In 79% of them, however, histologic study unmasked concealed pathologic substrates such as focal myocarditis or cardiomyopathy and conduction system diseases. A total of 16 victims (6%) had no evidence of structural heart disease and the mechanism of their SCD remained unexplained.