Ventricular hypertrophy, the only indisputable phenotypical marker of hypertrophic cardiomyopathy, is the basis of the physiopathology and treatment of the disease. Mitral valve abnormalities are usually considered to be secondary to the hypertrophy but the genesis of systolic anterior motion by the Venturi effect has been questioned by many clinical and experimental observations. Abnormalities of the mitral valve apparatus may in themselves (elongation of the valves, antero-internal malposition of the mitral papillary muscles and/or hyperlaxicity of antero-internal corhdae tendinae) create systolic anterior motion or subaortic obstruction in the absence of septal hypertrophy and/or increased subaortic flow velocities. Anatomo-clinical studies have confirmed this hypothesis: a high prevalence of mitral valve disease (increased valvular surface and length) has been found in hypertrophic cardiomyopathy. The recognition of these abnormalities is of value from the therapeutic (mitral valvuloplasty) diagnostic (in borderline cases) and genetic (when the primary nature of the abnormalities is confirmed) points of view.