The initiation of corrosion fatigue cracks has been studied for mild steel tested in 0.6 M sodium chloride solution. The results indicate that cyclic stress enhances the formation and coalescence of micropits at, and around, the inclusions, leading to the nucleation of cracks. Dissolution of the inclusions promotes local transport of hydrogen into the metal. Hydrogen plays a significant role in the initiation of voids and microcracks. Crack initiation also occurs with the formation of voids in and around slip bands, at grain boundaries and in the pearlitic regions.