One hundred myopic children between the ages of 8 and 13 years were fitted with Paraperm O2plus silicone-acrylate contact lenses. After 3 years of lens wear, the mean increase in myopia for the 56 subjects remaining in the study was 0.48 D (+/- 0.70) D as compared with 1.53 (+/- 0.81) D for a group of spectacle-wearing myopes matched for initial age and initial refractive error. The mean change in corneal refracting power for the contact lens wearers was a decrease (corneal flattening) of 0.37 (+/- 0.32) D. Assuming that little or no corneal change would have occurred in the absence of the contact lenses, we may conclude that corneal flattening (as measured by the keratometer) accounts for less than half of the effect of contact lenses in controlling myopia progression. A possible explanation for this disparity is that although the keratometer provides a valid measurement of corneal refracting power for a "normal" cornea, it fails to provide a valid measurement for a cornea that has been flattened by wearing a contact lens.