Background - Yield of causative variants in hypertrophic cardiomyopathy (HCM) is increased in some probands, suggesting different clinical subgroups of disease occur. We hypothesized that a negative family history and no sarcomere mutations represent a nonfamilial subgroup of HCM. We sought to determine the prevalence, natural history, and potential clinical implications of this nonfamilial subgroup of HCM. Methods and Results - Four hundred and thirteen unrelated probands with HCM seen in a specialized HCM center between 2002 and 2015 and genetic testing performed were included in this retrospective cohort study. There were 251 (61%) probands with no reported family history of HCM, including 166 (40% of total) probands with no sarcomere mutation, that is, nonfamilial HCM. Quantified family pedigree data revealed no difference in mean number of first-degree relatives screened between nonfamilial and sarcomere-positive groups. Adjusted predictors of nonfamilial status were older age (odds ratio, 1.04; 95% confidence interval, 1.02-1.06; P=0.0001), male sex (odds ratio, 1.96; 95% confidence interval, 1.11-3.45; P=0.02), hypertension (odds ratio, 2.80; 95% confidence interval, 1.57-5.00; P=0.0005), and nonasymmetric septal morphology (odds ratio, 3.41; 95% confidence interval, 1.64-7.08; P=0.001). They had a less severe clinical course with greater event-free survival from major cardiac events (P=0.04) compared with sarcomere-positive HCM probands. Genotype prediction scores showed good performance in identifying genotype-positive patients (area under the curve, 0.71-0.75) and, in combination with pedigree characteristics, were further improved. Conclusions - Approximately 40% of HCM probands have a nonfamilial subtype, with later onset and less severe clinical course. We propose a revised clinical pathway for management, highlighting the role of genetic testing, a detailed pedigree, and refined clinical surveillance recommendations for family members.