Purpose: To investigate the change of pupil diameter following cataract surgery or after-cataract surgery and the dependence of this change on the opacity at photopic and scotopic adaptation. Methods: Thirty-five eyes with cataract and 32 eyes with after-cataract were evaluated prospectively. The pupil diameter was measured at 220 lux (photopic) and 0.05 lux (scotopic) using a pupillometer. Cataract patients were examined preoperatively, and at 1 week and 2 months postoperatively, and after-cataract patients were examined preoperatively, and at 1 week postoperatively. The lens opacity was classified as opacity degree and the after-cataract was divided into pearl and fibrosis type. Results: Pupil diameter of cataract patients was reduced after surgery at photopic and scotopic adaptation (p<0.05). The pupil diameter of eyes with severe opacity degree was 4.3±1.1 mm and 5.0±1.1 mm at photopic and scotopic adaptation, respectively, and was larger than eyes with mild opacity degree (3.9±1.1 mm and 4.5±1.1 mm, respectively, p<0.05). At photopic and scotopic adaptation, pupil diameter of after-cataract patients was reduced by Nd:Yag laser (p<0.05). The pupil diameter of eyes with fibrosis type, 4.6±1.0 mm, was significantly smaller than that of eyes with pearls type (5.1±1.1 mm, p<0.05). Conclusions: The pupil diameter of eyes with severe opacity degree was larger that of eyes with mild opacity degree. In after-cataract patients, pupil diameter was significant different depending on opacification type at scotopic adaptation and it was reduced after Nd:Yag laser.