Hippus is described here as a sustained oscillation of the pupil, with a period of about 5 sec. The amplitude is about 1 mm, and the average diameter is decreased by 1-2 mm. We found Hippus in almost every subject that we tested, Leiden, Stenfert Kroese. Usually, Hippus seems to occur spontaneously, but it is more prone to appear when subjects are relaxed and passive. A repeated light step and a repeated accommodation may induce the occurrence of Hippus. Mental activity causes an immediate disappearance of Hippus. To prevent Hippus from contaminating pupillary measurements, a continuous monitoring of the pupil seems desirable.