IMPORTANCE: Sarcomere mutations and left ventricular (LV) hypertrophy (LVH) are cardinal features of hypertrophic cardiomyopathy (HCM). However, little is known about the full spectrum of phenotypic manifestations or how LVH influences disease expression. OBJECTIVES: (1) To characterize and assess phenotypic burden in sarcomere mutation carriers (genotype positive [G+]) and (2) to investigate the correlation between LV wall thickness (LVWT) and other disease features in mutation carriers. DESIGN, SETTING, AND PARTICIPANTS: This investigation was a cross-sectional, multicenter observational study in the setting of the HCMNet network of HCM clinical centers. Mutation carriers with LVH (G+/LVH+), mutation carriers without LVH (G+/LVH−), and healthy related control individuals (G−/LVH−) were enrolled through HCMNet sites. A total of 193 participants were enrolled and underwent study procedures. Participants were enrolled between April 9, 2010, and January 30, 2012. Study analysis was performed between June 2015 and May 2016. EXPOSURES :The primary stratifying variables were the presence of a sarcomere mutation and measures of LVWT. MAIN OUTCOMES AND MEASURES: Variables from standardized exercise testing, echocardiography, cardiac magnetic resonance imaging, serum biomarker measurement, and electrocardiography were compared across study cohorts. RESULTS: Analyses were performed in 178 participants, including 81 G+/LVH+ (mean [SD] age at baseline, 27  years), 55 G+/LVH− (20  years), and 42 G−/LVH− (18  years). All mutation carriers had smaller LV cavity, higher ratio of LVWT to diastolic diameter, and higher echocardiographic LV ejection fraction than controls. A phenotypic burden score was evaluated as the cumulative number of 7 traits (changes on electrocardiography; decreased LV systolic, diastolic diameter, or septal E′ velocity; higher ratio of LVWT to diastolic diameter; serum troponin level; and natriuretic peptide level) in each individual. The mean (SE) phenotypic burden was 4.9 (0.2) phenotypes per individual in G+/LVH+, 2.4 (0.2) in G+/LVH−, and 1.3 (0.2) in controls (P < .001). Classification and regression tree analysis identified an LV end-diastolic dimension z score less than −1.85 or the combination of an LV end-diastolic dimension z score of −1.85 or higher and a septal E′ velocity z score less than −0.52 as having 74% accuracy in discriminating G+/LVH− participants from controls. In mutation carriers, clinical variables demonstrated a continuous correlation with LVWT, generally without a clear cutoff signifying pathologic transition. CONCLUSIONS AND RELEVANCE: G+/LVH− individuals demonstrated altered cardiac dimensions and function and a higher burden of early phenotypes than healthy G− controls. Two methods discriminated phenotypic subgroups, namely, a sum across 7 traits and a regression tree–based rule that identifies constellations of distinguishing factors. Greater LVWT is associated with more prominent cardiac abnormalities in a continuous, although not always linear, manner. A single value of LVWT could not dichotomize the presence or absence of disease.