BACKGROUND: The prevalence, clinical characteristics and natural history of patients with hypertrophic cardiomyopathy (HCM) and midventricular obstruction (MVO) have not been adequately studied.METHODS AND RESULTS: A single-center cohort consisting of 423 patients (mean age, 49.3±17.2 years; 66.2% male) was thoroughly followed up for a median of 84 months (7 years; range, 6-480 months). MVO, characterized by the echocardiographic appearance of midventricular muscular apposition with a simultaneous mid-cavitary gradient ≥30mmHg, was identified in 34 patients (8%). Patients with MVO tended to be more symptomatic during their initial evaluation (>90% presented with NYHA class ≥II) compared to the rest of the HCM cohort. Apical aneurysm formation was identified in more than one-fourth of patients with MVO (26.5%), being a characteristic of the group. On multivariate Cox regression hazard analysis, presence of MVO strongly predicted progression to end-stage (burnt out) HCM and related heart failure (HF) deaths (hazard ratio, [HR], 2.62; 95% confidence interval [CI]: 1.2-8.8; P=0.047), as well as sudden death and associated lethal arrhythmic events (HR, 3.3; 95% CI: 1.26-8.85; P=0.016).CONCLUSIONS: MVO is a distinct phenotype of HCM associated with unfavorable prognosis in terms of end-stage HCM, sudden death and lethal arrhythmic events. The high adverse outcome rate necessitates early recognition of MVO and appropriate therapeutic interventions.