A model has been developed for the evolution of voids and dislocation loops during fast neutron irradiation of austenitic stainless steel. The model is based on a thermodynamic approach that calculates void nucleation and growth rates in terms of the supersaturation of vacancies and interstitials. It is recognized that the steady-state point-defect concentrations decrease with fluence as the result of the creation of additional sinks (voids and loops).The ability to monitor both the microstructural development and the steady-state concentrations of defects allows discussion of the in-pile mechanical properties. The yield strength of austenitic stainless steel is expected to increase rapidly during irradiation at 400°C due to the effectiveness of voids and dislocation loops as obstacles to dislocation motion. Irradiation at 600°C is predicted to result in a slowly increasing yield strength.In-reactor creep behavior is discussed in terms of a climb-controlled model for a dispersion strengthened system. Radiation-enhanced climb is expected to predominate at lower temperatures and stresses over the thermal climb component. Discussion of the possible effects of neutron flux and fluence on the in-pile steady-state creep rate is also included.