Introduction: Hypertrophic cardiomyopathy (HCM) is associated with high morbidity and mortality and remains the leading cause of sudden death in young people. Despite 5 decades of research, there is no disease modifying therapy for HCM. The cellular mechanisms leading to a proarrhythmic substrate are not yet fully elucidated but may be patient-specific. Here, we present single cell electrophysiology results of adult cardiomyocytes isolated from a cohort of HCM patients who underwent septal reduction therapy with surgical myectomy. Methods: Freshly isolated surgical specimens from HCM patients were enzymatically digested into single ventricular myocytes and compared to discarded wild-type (WT) donor hearts as controls. Myocytes displayed characteristic rod-shaped morphology, clear sarcolemmal ultrastructure, and intact plasma membranes without significant inclusions. After Ca 2+ readaptation and labeling with both a voltage and Ca 2+ sensitive fluorescent dye, cells were field stimulated at various pacing frequencies to obtain voltage and Ca 2+ tracings. APD restitution, response to drug therapy, and baseline data was compared to patient’s clinical data including electrocardiography, pathology, and multimodality imaging. Results: As compared to WT controls, HCM myocytes displayed significantly prolonged APD and Ca 2+ transients, as well as steeper APD restitution at all pacing frequencies tested. These results correlated with clinical QTc prolongation which was evident in HCM patients’ baseline ECGs that was not seen in controls. Despite genetic diversity in HCM samples, the HCM specimens appeared to share a common proarrhythmic phenotype. Finally, pathologic analysis of the myectomy specimens revealed evidence of myocyte hypertrophy, fibrosis, and cellular disarray, consistent with the clinical diagnosis of HCM. Conclusions: The single cell electrophysiology of HCM is markedly different than WT controls and displays a proarrhythmic phenotype. Single cell fluorescence imaging of a patient’s myocytes is a promising approach to elucidate patient specific electrophysiologic signatures and predict precision therapies for these patients.