Microstructural damage is measured in a stable austenitic alloy after nickel-ion bombardment to doses of 1 to 70 dpa at temperatures in the range of 840 to 1100 K. The influence of helium, both preimplanted at room temperature and coimplanted at a rate of 20 at. ppm per dpa, is examined. The helium causes considerable increases in the concentrations of cavities and reductions in cavity size, and shifts the peak swelling temperature upward by ∼50 K; growth of dislocation loops is delayed. Preimplanted helium has much more pronounced effects than coimplanted helium, including the generation of a large secondary population of small cavities deemed to be helium bubbles, and in some cases submicroscopic bubbles. Cavitation is assessed with regard to the concept of a critical size for bias-driven cavity growth. The results of this experiment are attributed to helium-enhanced cavity nucleation and to the influence of such nucleation on the cavity and dislocation sink strengths.