This study was conducted to verify, prospectively, the ability of an anatomical risk index (ARI) constructed from seven anatomical measures of cerebral volume and perisylvian asymmetry to predict reading ability in 43 children aged 9 to 18. We found that negative ARIs (low cerebral volume and symmetry) were associated with poor reading ability only in children with low processing speed. Regression analysis showed that anatomy, speed, and an interaction term predicted 53% of the variance in real word reading (p < .0001). Leftward perisylvian asymmetry and larger cerebral volumes may support cognitive flexibility in children with low processing speed.