Pressure gradients between atrium and left ventricle were measured in 5 of 17 patients with the diagnosis of hypertrophic cardiomyopathy. The diagnosis was confirmed by echocardiography and angiography. In 2 patients the hypertrophic cardiomyopathy was obstructive for the left ventricular outflow tract. Mitral stenosis was suspected in 3 out of the 5 patients who presented atrial fibrillation or frequent premature atrial beats and atrial overload phenomena of a third heart sound (which was misinterpreted as mitral opening snap) together with radiological enlargement of left atrium. Cardiac catheterization revealed middle pressure gradients between 7 and 14 mm Hg over mitral valve in 3 patients. In 2 patients significant early and middiastolic pressure gradients were recorded only after left ventricular angiography (volume load) was performed. Based on the data from cardiac catheterization, mitral stenosis was diagnosed in one patient, surgery revealed normal mitral leaflets. Echocardiography showed normally mobile mitral leaflets as well as signs of interventricular septal hypertrophy in all 5 patients. (In the patient who underwent surgery echocardiography was performed after the operation.) There were no echocardiographic signs of left ventricular inflow obstruction. The pathophysiologic mechanism of myocardial hypertrophy mimicking mitral stenosis is unknown. It may be suspected that third heart sound, diastolic murmur and the left ventricular inflow obstruction apparent from the pressure gradient are due to diminished myocardial compliance, direct mechanical effects of septal hypertrophy and decreased mobility of anterior mitral leaflet. Mimicking of mitral stenosis exists. Hypertrophic cardiomyopathy should therefore be considered in the differential diagnosis of mitral stenosis. Echocardiography may be helpful in evaluation.