Only 8 cases of hamartomas of mature cardiac myocytes have been reported. The aim of the study was to describe 7 new cases and provide clinicopathologic correlation. Our anatomical pathology database was searched for all cases of cardiac hamartoma, of which 7 represented mature myocyte type. Medical records were reviewed for clinical information, and microscopic slides were evaluated for extent of characteristics. Five males and 2 females ranged in age from 6 months to 74 years (mean, 23 years). There were 11 ventricular hamartomas (8 left free wall, 2 right free wall, 1 septum). Death in 3 infants was unrelated to incidental hamartomas discovered at autopsy. A 10- and 16-year-old were asymptomatic but had abnormal electrocardiogram (ECG) results, which led to detection of cardiac masses by imaging studies. Two adult males had only mild coronary disease angiographically. The 57-year-old, who died suddenly, had a 7-year history of abnormal ECG results. The 74-year-old, who died after aortic surgery, had a 3-year history of chest discomfort. Their hamartomas were identified at autopsy and contributed to sudden death in 1. Microscopically, all hamartomas were involved by myocyte hypertrophy and disarray, without inflammation or calcification. Myocyte vacuolization and venular dilatation occurred only in the pediatric cases, and interstitial adipose tissue only affected 1 adult. In conclusion, hamartomas of mature cardiac myocytes may be detected at any age. They primarily affect males, arise predominantly in the left ventricle, are asymptomatic, may have nonspecific ECG findings, and rarely may be associated with sudden death. Microscopic findings in infants differ from older patients.