Background: Cardiomyopathies are a group of diseases of the heart that can lead to heart failure, cardiac arrhythmia, and sudden death. They typically manifest as an enlarged heart or a normal heart with microscopic anomalies (fibrosis, inflammation, etc.). The aim of the study is to investigate the prevalence of deaths due to cardiomyopathies of unknown etiology in young subjects in a forensic pathology setting. Materials and Methods: Deaths due to cardiomyopathy in decedents less than 40 years old evaluated at the Cook County Medical Examiner’s Office in Chicago from January 2013 to June 2018 were studied. Results: In total, 140 cases of cardiomyopathies were identified in the study period: among these, in 20 cases (14%), no underlying etiology could be found through medical history and autopsy investigation. The demographics and the macroscopic and microscopic findings of these cases are described, highlighting the importance of medical history review and adequate histological sampling of hearts in cases of sudden, unexpected death in children and young adults. Conclusions: This study highlights the importance of medical history review and adequate histological sampling of hearts in cases of sudden, unexpected death in children and young adults to identify possible cardiomyopathies of unknown etiology.