BACKGROUND Hypertrophic cardiomyopathy (HCM) is the most common genetic heart disease characterized by a diverse clinical and phenotypic spectrum. This study reports the prevalence, morphology, clinical course, and management of an underrecognized subgroup of HCM patients with left ventricular apical aneurysms. RESULTS Of 1299 HCM patients, 28 (2%) were identified with left ventricular apical aneurysms, including a pair of identical twins. Aneurysms were recognized at a wide age range (26 to 83 years), including 12 patients (43%) who were <or=50 years of age. Apical aneurysms varied considerably in size (maximum dimension, 10 to 66 mm), were dyskinetic/akinetic with thin rims, and were associated with transmural (and often more extensive) myocardial scarring identified by late gadolinium enhancement cardiovascular magnetic resonance. Apical aneurysms were recognized by echocardiography in only 16 of 28 patients (57%) but by cardiovascular magnetic resonance in the 12 patients undetected by echocardiography. Left ventricular chamber morphology varied; however, 19 patients (68%) showed an "hourglass" contour, with midventricular hypertrophy producing muscular narrowing and intracavitary gradients in 9 patients (74+/-42 mm Hg). Sarcomeric protein missense mutations known to cause other phenotypic expressions of HCM were present in 3 patients. Over 4.1+/-3.7 years of follow-up, 12 patients (43%) with left ventricular apical aneurysms experienced adverse disease complications (event rate, 10.5%/y), including sudden death, appropriate implantable cardioverter-defibrillator discharges, nonfatal thromboembolic stroke, and progressive heart failure and death. CONCLUSIONS Patients with left ventricular apical aneurysms represent an underappreciated subset in the heterogeneous HCM disease spectrum with important clinical implications, often requiring a high index of suspicion and cardiovascular magnetic resonance for identification. Apical aneurysms in HCM are associated with substantial cardiovascular morbidity and mortality and raise novel treatment considerations.