Objective: To compare the use of contact lenses and intraocular lenses (IOLs) for the optical correction of unilateral aphakia during infancy. Methods: In a randomized, multicenter (12 sites) clinical trial, 114 infants with unilateral congenital cataracts were assigned to undergo cataract surgery with or without IOL implantation. Children randomized to IOL treatment had their residual refractive error corrected with spectacles. Children randomized to no IOL treatment had their aphakia treated with a contact lens. Main Outcome Measures: Grating acuity at 12 months of age and HOTV visual acuity at 41/2 years of age. Application to Clinical Practice: This study should determine whether either treatment for an infant with a visually significant unilateral congenital cataract results in a better visual outcome. Results: Enrollment began December 23, 2004, and was completed January 16, 2009. The median age at the time of cataract surgery was 1.8 months. Fifty patients were 4 to 6 weeksofageatthetimeofenrollment;32,7weeksto3months ofage;andtheremaining32, morethan3tolessthan7months of age. Fifty-seven children were randomized to each treatment group. Eyes with cataracts had shorter axial lengths and steeper corneas on average than the fellow eyes. Conclusions: The optimal optical treatment of aphakia in infants is unknown. However, the Infant Aphakia Treatment Study was designed to provide empirical evidence of whether optical treatment with an IOL or a contact lens after unilateral cataract surgery during infancy is associated with a better visual outcome. Trial Registration: clinicaltrials.gov Identifier: NCT00212134.