Characteristic echocardiographic features of hypertrophic obstructive cardiomyopathy were recorded in 24 patients, all of whom had asymmetric septal hypertrophy and systolic anterior motion of the mitral valve (SAM) at rest or after pharmacodynamic stimulation. The relationship between outflow tract obstruction and SAM was assessed by comparison with data obtained at cardiac catheterisation and external mechanography: SAM seems to be a non-specific phenomenon and may be recorded in cases of hypertrophic cardiomyopathy without obstruction during pharmacodynamic stimulation. In forms with obstruction, SAM and the severity of obstruction increase with the degree of spetal hypertrophy. The increased contractility of the left ventricular posterior wall appears to be an important factor in the mechanism of SAM which can be prevented by betablockade in moderate or labile forms. When SAM is permanent, whatever the gradient recorded, it is a sign of anatomical deformation of the left ventricle and may be an additional indication for cardiac surgery.