Recently, dual chamber pacing has been proposed as a new treatment for left ventricular outflow tract obstruction in patients with hypertrophic obstructive cardiomyopathy (HOCM). However, dual chamber pacing may be detrimental to both systolic and diastolic function of the left ventricle, particularly at short atrioventricular (AV) delay. A 45-year-old man and a 58-year-old woman with HOCM underwent transesophageal echocardiography and hemodynamic study at various AV delays (50-200 msec). In both patients, shortening of AV delay increased pulmonary capillary wedge pressure. The transesophageal echocardiographic study showed a remarkable shortening of the ventricular filling duration at atrial systole and an increase of pulmonary venous reversal at short AV delay (50-100 msec). Therefore, short AV delay was detrimental to diastolic filling and thus increased pulmonary capillary wedge pressure. Transesophageal echocardiographic study may be helpful for defining the optimal AV delay more precisely in HOCM during dual chamber pacing.