Ferritic stainless steel with 18 wt% of chromium is studied in order to clarify its sensitivity to hydrogen effects. Hydrogen uptake and diffusion were analyzed using thermal desorption method. Hydrogen diffusion transport and trapping in the concentrated Fe-Cr alloy is discussed. Hydrogen effects on the mechanical properties of the steel are studied with CERT performed under continuous H-charging. Hydrogen has a remarkable effect on the elongation to fracture of the Fe-Cr ferrite: uniform elongation of the steel reduces up to 75% in presence of hydrogen. A special attention is paid to the role of grain size on the steel sensitivity...