BackgroundTreatment of pediatric cataract remains challenging because of the extremely high incidence of postoperative adverse events (AEs), especially the AEs related to the locations of secondary implanted intraocular lens (IOL). There are two common locations for secondary IOL implantation in pediatric aphakic eyes: ciliary sulcus or in-the-bag implantation. However, there are currently no large, prospective studies comparing complication rates and visual prognosis of in-the-bag versus ciliarysulcus secondary IOL implantation in pediatric patients. Whether or how much secondary in-the-bag IOL implantation benefits the pediatric patients more than sulcus implantation and deserves to be performed routinely by surgeons remains to be elucidated. Here, we describe the protocol of a randomized controlled trial (RCT) designed to evaluate the safety and efficacy of two approaches of IOL implantation in pediatric aphakia.MethodsThe study is a multicenter, single-blinded RCT with 10 years of follow-up. Overall, a minimum of 286 eyes (approximately 228 participants assuming 75% have two study eyes) will be recruited. This study will be carried out in four eye clinics across China. Consecutive eligible patients are randomized to undergo either secondary in-the-bag IOL implantation or secondary sulcus IOL implantation. Participants with two eyes eligible will receive the same treatment. The primary outcomes are IOL decentration and the incidence of glaucoma-related AEs. The secondary outcomes include the incidence of other AEs, IOL tilt, visual acuity, and ocular refractive power. Analysis of the primary and secondary outcomes is to be based on the intention-to-treat and per-protocol analysis. Statistical analyses will include theχ2test or Fisher’s exact test for the primary outcome, mixed model and generalized estimated equation (GEE) model for the secondary outcome, Kaplan–Meier survival curves for the cumulative probability of glaucoma-related AEs over time in each group.DiscussionTo the best of our knowledge, this study is the first RCT to evaluate the safety and efficacy of secondary IOL implantation in pediatric aphakia. The results will provide high-quality evidence for the clinical guidelines for the treatment of pediatric aphakia.Trial registrationClinicalTrials.gov NCT05136950. Registered on 1 November 2021.