Fiber-reinforced polymer (FRP) overlays have been successfully used in the aerospace industry to repair fatigue damage in aluminum plates. With this success there is potential for use of similar FRP overlays to repair fatigue damage in aging steel bridge structures. This study investigated the effectiveness of repairing fatigue damage in steel plate with adhesively bonded carbon fiber-reinforced polymer (CFRP) overlays. A total of 15 steel plate specimens with preexisting fatigue cracks were repaired with varying thicknesses of CFRP overlays to evaluate the effect of the ratio of axial stiffness of the composite to that of the underlying steel, the axial stiffness ratio (SR), on increased fatigue life and decreased applied stress. The results showed that increasing the axial stiffness ratio from 0 to 0.4 could increase the fatigue life by a factor of 10 for the most extreme conditions, and with an optimal axial stiffness ratio infinite fatigue life may be reached. Fatigue life of the steel specimens in this study was found to be dependent on both axial stiffness and applied stress range. Results from finite-element analyses validated the use of axial stiffness as a design parameter and correlated to the experimental results discussed. (C) 2013 American Society of Civil Engineers.