Micro-scale damage with a topographical contrast has been observed in cold-worked Nb-stabilised 20%Cr-25% Ni stainless steel, following irradiation with 2.2 MeV protons at 400 degrees C and a dose rate of similar to 10(-5) dpa/s. After an irradiation dose of 3 and 5 dpa, microstructural changes were found to a depth of 22 mu m below the irradiated surface, coincident with the estimated mean range of protons in the material as predicted by the TRIM code. Large variations of intragranular mis-orientations, in combination with a depletion of chromium at grain boundaries, were observed in the proton-irradiated layer. This was accompanied by an increase in nano-hardness, which was linked to a partial transformation of tangled dislocation networks into dislocation loops. Such visible microstructural changes have not been reported before in the absence of post-irradiation annealing treatments.