PURPOSE.: To describe corneal and crystalline lens dimensions before, during, and after myopia onset compared with age-matched emmetropic values. METHODS.: Subjects were 732 children aged 6 to 14 years who became myopic and 596 emmetropic children participating between 1989 and 2007 in the Collaborative Longitudinal Evaluation of Ethnicity and Refractive Error Study. Refractive error was measured using cycloplegic autorefraction, corneal power using a hand-held autokeratometer, crystalline lens parameters using video-based phakometry, and vitreous chamber depth (VCD) using A-scan ultrasonography. Corneal and crystalline lens parameters in children who became myopic were compared with age-, gender-, and ethnicity-matched model estimates of emmetrope values annually from 5 years before through 5 years after the onset of myopia. The comparison was made without and then with statistical adjustment of emmetrope component values to compensate for the effects of longer VCDs in children who became myopic. RESULTS.: Before myopia onset, the crystalline lens thinned, flattened, and lost power at similar rates for emmetropes and children who became myopic. The crystalline lens stopped thinning, flattening, and losing power within ±1 year of onset in children who became myopic compared with emmetropes statistically adjusted to match the longer VCDs of children who became myopic. In contrast, the cornea was only slightly steeper in children who became myopic compared with emmetropes (<0.25 D) and underwent little change across visits. CONCLUSIONS.: Myopia onset is characterized by an abrupt loss of compensatory changes in the crystalline lens that continue in emmetropes throughout childhood axial elongation. The mechanism responsible for this decoupling remains speculative but might include restricted equatorial growth from internal mechanical factors. Copyright © 2012 American Academy of Optometry.