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We report a 15-year-old patient who presented with exercise dyspnea and limitation of physical activity. Echocardiography revealed significant left ventricular outflow tract obstruction caused by systolic anterior motion (SAM) of the mitral valve. The wall thickness of the left ventricle was within normal limits. Elongation of the mitral leaflets and anterior displacement of the posteromedial papillary muscle were apparent in the echocardiographic examination. These two factors have been previously demonstrated to play a central role in the occurrence of SAM in patients with hypertrophic cardiomyopathy. The present case validates that such intrinsic abnormalities of the mitral valve can cause significant SAM even in the absence of left ventricular hypertrophy.