ABSTRACT: Heart failure continues to be a leading cause of mortality and morbidity. Understanding the basic mechanisms of heart regeneration in humans and stimulating them would improve the lives of many patients. Proliferation of heart muscle cells (cardiomyocytes) is essential for heart development and regeneration. After birth, the proliferative potential of cardiomyocytes declines, but neonatal mammalian hearts are thought to maintain the ability to regenerate until cardiomyocytes undergo permanent cell cycle arrest, establishing a barrier for regeneration. Strategies for stimulating cardiomyocyte proliferation are under development and would be most effective when initiated before cardiomyocytes withdraw permanently from the cell cycle. However, the point at which cardiomyocyte proliferation ceases in humans is controversial; some studies place this point before birth, some in the first year after birth, and our data demonstrate cardiomyocyte proliferation extending well into childhood, i.e., the first 10 – 20 years of life. This picture is further complicated by our published and unpublished results suggesting that infants with congenital heart disease (CHD) have decreased cardiomyocyte proliferation. Resolving how age and heart disease alter the temporal pattern of cardiomyocyte cell cycle withdrawal is an essential problem in cardiac biology. Improved understanding of cardiomyocyte proliferation will be critical for developing new regenerative strategies to prevent and treat heart failure in patients with cyanotic CHD, the most common birth defect affecting 0.3% of newborns. We will determine when cardiomyocyte proliferation decreases in pediatric patients by investigating two leading cardiac diseases: tetralogy of Fallot (ToF), the most common form of cyanotic congenital heart disease (CHD), which affects 0.3% of newborns, and dilated cardiomyopathy (DCM), the most common cause for heart transplantation in the pediatric age group. Our central hypothesis is that cardiomyocyte proliferation shows an age- and disease-dependent decline. We will enroll neonates, infants, and children to quantify cardiomyocyte proliferation. We have formed a research team consisting of pediatric cardiologists and cardiac surgeons, pathologists, and basic scientists. The critical element of our approach is the use of a highly innovative method in which we will label patients with innocuous thymidine carrying stable isotope markers (15N-thymidine and 2H- thymidine). Cells in S-phase of the cell cycle incorporate thymidine into their DNA, and their offspring retain the label for at least 6 cell divisions. We will visualize the isotope in post-surgical tissue samples with an innovative method: Multi-isotope imaging mass spectrometry (MIMS) to quantify cardiomyocyte proliferation and differentiation. Preliminary data from our first study patient with ToF provide first-in-human results and validate our approach.