Pupil centration is important to the optical blur on the retina. Using a dual Maxwellian view system we measured the centration of the pupil with respect to the achromatic axis of the eye as a function of pupil size. Significant shifts of the pupil center (up to 0.6 mm) with pupil dilation were measured in both nasal and temporal directions. The effect was usually symmetrical between the two eyes and the shift was linear with pupil size in one-half of the subjects. From their initial positions the linear pupil center shifts with dilation were in the direction of the achromatic axis.