Abstract Elastic–plastic fracture mechanics experiments were conducted on three heats of 21Cr–6Ni–9Mn austenitic stainless steel (two forgings and one annealed material), emphasizing the effects of high concentrations of thermally precharged hydrogen as well as crack-growth orientation and ferrite content. Hydrogen was the dominant variable in this study, causing a reduction in fracture initiation toughness of greater than 80% in the forgings independent of crack-growth orientation and ferrite content. Reflecting the fracture toughness measurements, hydrogen also caused dramatic changes in fracture modes. Microscopy evidence indicated that fracture in hydrogen-precharged materials was governed by localized deformation. The impingement of deformation bands on obstacles such as boundaries or intersecting deformation bands caused stress concentrations, leading to void or microcrack formation at these sites. It is postulated that void or microcrack formation can be solely attributed to hydrogen-enhanced localized deformation without invoking any mechanism where hydrogen directly lowers the fracture resistance.