PURPOSE To evaluate the theoretical influence of the change in corneal asphericity (Delta Q) on the change in fourth-order Zernike spherical aberration coefficient (Delta C-0(4)) with customized aspheric refractive correction of myopia and hyperopia. METHODS The initial anterior corneal surface profile was modeled as a conic section of apical radius of curvature R-0 and asphericity Q(0). The postoperative corneal profile was modeled as a conic section of apical curvature R-1 and asphericity Q(1), where R-1 was computed from defocus D, and Q(1) selected for controlling the postoperative asphericity. The corresponding change in fourth-order spherical aberration (Delta C-0(4)) was computed within a 6-mm optical zone using inner products applied to the incurred optical path changes. These calculations were repeated for different values of D, R-0, Q(0), and various intended asphericityC(0)(4) values. RESULTS Increasing negative spherical aberration (asphericityC(0)(4) < 0) requires a change toward more negative values of asphericity (increased prolateness; Delta Q < 0) for hyperopic and low myopic corrections, but more positive values (Delta Q < 0) for high myopic correction. The larger the intended change in corneal spherical aberration (Delta C-0(4)), the more myopic the threshold value for which the required change in asphericity, Delta Q, becomes positive. The influence of the magnitude of paraxial defocus correction is less pronounced when larger changes in C-0(4) are intended. CONCLUSIONS These results provide a basis for controlling the direction (sign) and the magnitude of spherical aberration changes when using customized aspheric profiles of ablation.