Purpose To observe the changes of pupil size over time after Visian Implantable Collamer Lens (ICL) implantation. Patients and methods We retrospectively examined 30 eyes of 23 consecutive patients undergoing ICL implantation. We measured the entrance and real pupil diameters using a Hartmann-Shack aberrometer (KR-9000, Topcon, Tokyo, Japan) before and 1 day, 1 week, and 1, 3, 6, and 12 months after surgery. We also investigated its relationship with the amount of vaulting using slit-lamp microscopy 1 year postoperatively. Results The entrance pupil diameters were 6.24 ± 0.66 (mean ± standard deviation) mm preoperatively, and 5.53 ± 0.69, 6.18 ± 0.61, 6.21±0.80, 6.29±0.74, 6.23±0.76, and 6.40±0.70 mm, 1 day, 1 week, and 1, 3, 6, and 12 months postoperatively, respectively; and the respective real pupil diameters were 5.44 ± 0.55 mm preoperatively, and 4.95 ± 0.60, 5.53 ± 0.52, 5.55 ± 0.69, 5.63 ± 0.64, 5.57 ± 0.64, and 5.72 ± 0.60 mm, 1 day, 1 week, and 1, 3, 6, and 12 months postoperatively. Pupil diameters and the amount of vaulting were not significantly associated (Pearson correlation coefficient r ¼ 0.14, P ¼ 0.45 for entrance pupil, r ¼ 0.13, P ¼ 0.49 for real pupil). Conclusions Both pupil diameters decreased transiently 1 day after ICL implantation, but soon recovered to the preoperative level, indicating that intraoperative mechanical irritation of the uveal tissue and the early postoperative inflammatory response may induce transient decreases in pupil diameter, and that this surgical technique alone with appropriate ICL size selection probably induces no significant pupil diameter change.