Resistance to fatigue crack propagation of pre- and postirradiation AISI Types 304 and 316 stainless steels was determined at 800 and 1100 F (427 and 593 C) using the fracture mechanics approach. The effect of irradiation on fatigue resistance was dependent upon test temperature and irradiation conditions. In general, irradiation degraded fatigue resistance at 1100 F (593 C) but at 800 F (427 C) enhancement as well was observed. In both steels irradiated in a thermal reactor to a fluence of 1.8 x 1021 n/cm2 >0.1 MeV, fatigue crack growth rates at 800 F (427 C) were lower than in the unirradiated steels for a given stress intensity factor range (?K). However, at 1100 F (593 C) the effect was reversed and crack growth rates were higher in the irradiated steels. Irradiation in a fast reactor to a fluence of ~1.2 x 1022 n/cm2 >0.1 MeV caused fatigue crack growth rates at 800 F (427 C) to increase at low values of ?K and decrease at high values of ?K. At 1100 F (593 C) the crack growth rates in the irradiated steel were either the same as or higher than in the unirradiated steel. The influence of irradiation on fatigue life generally reflected the effect observed on crack growth rate.