BACKGROUND: Hypoplastic Left Heart Syndrome is a congenital birth defect that is defined by underdevelopment of the left heart during pregnancy. This is especially dangerous as the left heart holds the systemic flow of blood- the oxygenated blood. Not enough oxygen throughout the whole body causes cyanosis, which symptoms include bluish discoloration of the skin or mucous membrane due to low oxygen saturation. Single Ventricle Palliation followed by Biventricular Conversion is the most common surgical procedural pathway to correct this defect. The goal is to convert from a single ventricle circulation during single ventricle palliation to biventricular circulation via biventricular conversion, which is the normal heart anatomy. Single Ventricle Pallation consists of three stages: Stage 1 Norwood Procedure, Bidirectional Glenn, and Fontan. Biventricular Conversion can be performed after any of the three stages. In addition, further compromise of the left ventricle includes other factors such as a thickening of fibroblast-like cells on the endocardial layer called endocardial fibroelastosis. Therefore, additional surgical procedures, also known as recruitment procedures, combat these problems. It is critical to find a correlation between a specific procedure and post surgery success in left ventricle growth and function for these patients. OBJECTIVES: Patients with Hypoplastic Left Heart Syndrome at Boston Children’s Hospital have undergone single ventricle palliation with some patients proceeding to biventricular conversion. This study aimed to study the palliation stages individually and recruitment procedures (specifically endocardial fibroelastosis resection) on the effect of left ventricle growth. METHODS: Patients with Hypoplastic Left Heart Syndrome were studied retrospectively (before 2014) and prospectively (after 2014 until December 1, 2018). Single Ventricle Palliation and Biventricular Conversion were analyzed via descriptional analysis with evidence of left ventricular growth measured by left ventricular end diastolic volume and respective z-scores. Z-scores were used to standardize end diastolic volume values across variability in age, weight, and height. RESULTS: A total of 55 patients underwent single ventricle palliation and 39 ended with biventricular circulation via biventricular conversion. Overall, there was a 9.29 ml increase in end diastolic volume between Bidirectional Glenn and Fontan and a 0.795 increase in end diastolic volume z-score between Fontan and Biventricular Conversion. Next, those who did not have recruitment procedures experienced a 135.6%, 48.8%, and 0% growth at Stage 1, Bidirectional Glenn, and Fontan, respectively, before directly proceeding to biventricular conversion. Those with recruitment experienced a 44.5%, 90.4%, and 83.0% growth at Stage 1, Bidirectional Glenn, and Fontan, respectively, before directly proceeding to biventricular conversion. Finally, there was a 50.2% and 62.3% in left ventricular growth at Bidirectional Glenn and Fontan, respectively, after endocardial fibroelastosis resection compared to only a 6.9% growth at Stage 1. CONCLUSION: Bidirectional Glenn was the most effective palliation stage for left ventricular growth. Recruitment in patients at this stage was associated with growth that exceeds those who did not have recruitment. This stage also best demonstrates the ability and success of growing a small ventricle to be adequate for biventricular conversion. Left ventricular growth at Fontan circulation holds promising results that are a point of interest for more studies. Endocardial Fibroelastosis resection is more effective on left ventricular growth at Bidirectional Glenn and Fontan compared to Stage 1.