This report describes the simultaneous occurrence of mitral valve prolapse (MVP) and systolic anterior motion (SAM) in hypertrophic cardiomyopathy (HC). In 25 patients (aged 7 to 62 years, mean 29), 15 (60%) of whom were male, distal portions of the anterior or posterior mitral leaflets approached or made midsystolic contact with the ventricular septum, whereas the proximal portion of the mitral leaflets showed marked cephalad excursion into the left atrium, 5 to 15 mm beyond the mitral annular plane. Three mitral valves that were available for gross visual inspection were not morphologically typical of patients with primary MVP. Clinical features and natural history (1 to 14 years [mean 6] of follow-up), cardiac dimensions, and distribution of left ventricular hypertrophy defined in the study patients did not appear to differ distinctly from those in the overall referral population of patients with HC evaluated at our institution. Hence, patients with HC may show a striking pattern of mitral valvular motion involving SAM into the left ventricular outflow tract, as well as MVP; this prolapse motion is probably due to anatomic disproportion between the mitral valve and the small left ventricular cavity rather than to the coexistence of 2 separate disease entities. Such patients further define the great diversity evident within the broad clinical spectrum of HC.