The authors report postoperative change in pupil diameter and pupil dynamics after phaco-emulsification-aspiration and intra-ocular lens implantation by comparing the operated eye and non-operated eye in 20 patients who received unilateral surgery. Miosis was noted in the operated eye throughout the postoperative period in the dark, and also in the light at an early stage after the operation. Both the maximal constriction velocity and maximal dilatation velocity of the light reflex decreased postoperatively, but the constriction velocity alone was accelerated 1 or 2 weeks after the operation. In the drug instillation test, pupillary responses to both neosynephrine and pilocarpine were decreased, indicating damage both to the dilator and sphincter muscles. Since no hypersensitivity of the pupil was noted, lesions in the axonal fibres and denervation were judged to be absent. Postoperative miosis is believed to be explained by irreversible damage to the dilator muscle, as well as by the transient effects of inflammatory substances.