Horizontal positional nystagmus due to peripheral vestibular lesions might be caused by unilateral dysfunction of the utricular apparatus. To determine whether horizontal positional nystagmus is caused by utricular dysfunction alone, we examined the dynamic response of positional nystagmus in eleven patients with positional vertigo when the head was rotated from the supine position to the left and to the right. Positional nystagmus varied with the speed and the direction of the head rotation in all cases. Positional nystagmus was probably provoked not only by the head position but also by the acceleration or the velocity of head rotation. A recent study indicated that horizontal nystagmus induced by off-vertical axis rotation is due to utricular changes and is related to the velocity of head rotation. Therefore, it may be that horizontal positional nystagmus is not necessarily provoked by an interaction between the lateral canal and the utricle, but by utricular dysfunction alone.