The impact of the duration of atrioventricular (AV) delay on obstruction in hypertrophic cardiomyopathy was evaluated in 12 patients by cardiac catheterization, and in 8 of them also by Doppler echocardiography. The AV delay was programmed in random order at 125, 100 and 75 ms in the invasive study and at 120, 100 and 80 ms after pacemaker implantation. The arterial pressure did not changed throughout the studies, whereas the gradient decreased significantly by reducing the AV delay value; the greater gradient reduction was obtained, in both studies, with the AV delay set between 75 or 80 and 100 ms. QRS duration increased significantly by reducing the AV delay during both studies. The widest QRS was not associated with the smallest gradient in all patients. Changes in gradients were similar during the invasive and noninvasive protocols.