The effect of irradiation to fluences in the range 1.2 to 2.7 × 1022 n/cm2 (>0.1 MeV) on the microstructure and creep-rupture properties of annealed and cold-worked Type 316 stainless steel has been examined for irradiation temperatures in the range 450 to 825 C (842 to 1517 F). For irradiation in the annealed condition below about 650 C (1202 F), voids and dislocations were formed. Cold working before irradiation suppressed void formation at all temperatures except 580 C (1076 F) where voids were occasionally found in recovered areas. Extensive precipitation occurred at temperatures above about 500 C (932 F), and the amount and type of precipitate were sensitive to the initial microstructure. Changes in rupture life and creep rate were observed, but the most significant effect of irradiation was a large decrease in ductility resulting from enhanced intergranular fracture. The grain boundary cracks were formed by the linking of cavities on those boundaries nearly normal to the applied stress. The experimental results are most consistent with a model in which nucleation of the cavities occurs on helium bubbles with growth controlled by stress-enhanced diffusion of vacancies. Experimental evidence also indicates that the strength of the matrix influences the crack propagation rate.