Microstructures of helium preinjected, pressurized tube specimens of 22% cold-worked type 316 stainless steel have been examined after neutron irradiation in EBR-II at 525°C to a dose of 23 dpa under a stress of 32 MPa. Preinjected helium at 20 and 60 appm suppressed the swelling. The degree of suppression increased with increasing amounts of preinjected helium. The uninjected sample areas contained a unimodal distribution of cavities, including the largest cavities observed. A bimodal cavity distribution containing a large number density of small cavities and a smaller number density of larger cavities was found. The large number of fine cavities found in the preinjected sections are believed to change the ratio of cavity to dislocation sink strengths and to reduce the bias, resulting in a reduction of the cavity growth rate. The principal precipitate formed during irradiation of the stressed tube specimens was eta phase.