We have used a second-order perturbation treatment of the Dirac-Coulomb-Hartree-Fock method to estimate relativistic contributions to energies in the G2/97 test set. The one-electron relativistic effect on atomization energies of molecules containing first-and second-row atoms nearly always reduces binding. When the relativistic corrections are included in G3 theory and assessed on the G2/97 test set, there is little change in overall performance.