A compliance change was observed during fatigue testing of ASTM A710 HSLA steel using constant "K" CDCB specimen. The compliance decreased from 1.296 x 10(-5) mm/N to 1.235 x 10(-5) mm/N when the environment was changed from hydrogen to air under the fatigue test conditions of f = 0.2 Hz, R = 0.1 and Delta K = 10 MPa root m. The compliance change was observed in all fatigue testing while changing the environment from hydrogen to air. This compliance change can be explained numerically using the differential method for the design factors of the CDCB specimen. It was found from the calculation that the compliance change corresponded to a 6.3% change in Young's modulus. It is proposed that the increased compliance resulted from the decreased Young's modulus, the reduced Young's modulus resulted from the increased lattice dilation which in turn resulted from a significantly increased hydrogen concentration at the crack tip region. The increased hydrogen concentration at the crack tip resulted from stress-induced hydrogen diffusion at the crack tip region. (c) 2005 Springer Science + Business Media, Inc.