Hypertrophic cardiomyopathy (HC) is associated with a well-recognized risk for unexpected sudden death (SD). Most such reported patients have been referred to dedicated centers and/or expert cardiologists for risk stratification, with the number of SDs decreasing sharply due to penetration of the implantable cardioverter-defibrillator (ICDs) into HC practice. However, the clinical circumstances, and morphologic features of HC patients who incur SD without the opportunity to be considered for preventive intervention with ICDs are largely undefined. Using the long-standing unique Jesse Edwards Registry (St. Paul, Minnesota), we studied 86 selected heart specimens from young HC patients who died suddenly and unexpectedly without prior clinical evaluation, ages 31 ± 16 years. The patients were predominantly male (87%) with only modest phenotypic expression and maximum LV wall thickening of only 18 ± 4 mm. SD events occurred predominantly with sedentary/mild activities (66%) often in bed or asleep (32%), but also during physical activity (22%) including with organized competitive sports. This largely unappreciated sub-population of patients with HC (and SD) is characterized by mild-to-moderate degree of LV hypertrophy, representing a clinical challenge which is particularly relevant in the current ICD era for HC, with the potential for SD prevention.