A normalized-and-tempered Fe-9Cr-1Mo steel, modified with small quantities of niobium and vanadium, was bombarded with 4 MeV iron ions to a nominal displacement level of 100 displacements per atom (dpa) at temperatures of 400, 450, 500, 550, and 600°C. The major microstructural damage feature created in the lathlike α-ferrite grains was dislocation tangles that coarsened with increasing bombardment temperature. Sparse cavities were heterogeneously distributed at 500 and 550°C. Incorporation of helium and deuterium simultaneously in the bombardments at rates of 10 and 45 atomic parts per million (appm)/dpa, respectively, introduced very high concentrations of small cavities at all temperatures, many of them on grain boundaries. These cavities were shown to be promoted by helium. A small fraction of the matrix cavities exhibited bias-driven growth at 500 and 550°C, with swelling <0.4%. This is a very narrow temperature range for bias-driven swelling. It is about 125°C higher than the peak swelling temperature found in neutron irradiations, which is compatible with the higher damage rate used in the ion bombardments. High concentrations of subgrain boundaries and dislocations resulting from the heat treatment, and unbalanced cavity and dislocation sink strengths in the damage structures contribute to the swelling resistance. Such resistance may not be permanent. High densities of bubbles on grain boundaries indicate a need for helium embrittlement tests.