The Pulfrich stereo-illusion effect occurs in patients with monocular anterior visual system disease when viewing a pendulum. It also can be induced by placing a monocular neutral-density filter in a normal subject. We studied 10 normal subjects to determine if the Pulfrich effect could be induced by monocular illumination in addition to monocular neutral-density filtration. Eighteen of 20 eyes observed the Pulfrich effect with monocular illumination, but the rotation of the ellipse was opposite in direction to that of the neutral-density filter. Our data suggests that the Pulfrich effect results from intereye luminance disparity.