Annealed Type 304 and annealed and 20 percent cold-worked Type 316 steels have been bombarded with 5 MeV nickel ions at temperatures from 525 to 725 C (977 to 1337 F) and damage levels up to 280 dpa. The peak swelling temperatures are 150 and 115 C (270 and 207 F) higher than the peak swelling temperatures in reactor for Type 304 and 316, respectively. Swellings of over 50 volume percent have been produced in both alloys at the peak swelling temperature. Twenty percent cold-worked Type 316 does not swell appreciably less than the solution-treated material. Comparison of in-reactor and ion-produced swellings, and void sizes and void densities, indicate that the nickel ion bombardment provides an acceptable simulation of fast-reactor damage. The high-dose ion results are used to guide the extrapolation of reactor data to higher fluences, leading to predictions of over 40 percent swelling in annealed Type 304 at 2 × 1023 n/cm2 (fast) and in annealed Type 316 at 3 × 1023 n/cm2.