This investigation determined the combined effect of water vapor and repealed loading on slow crack growth and fracture toughness. Round notched bars of SAE 4340 steel with a Rockwell C hardness of 48 were tested in tension, in a controlled environment, and at room temperature. The compliance of the specimen was measured on each cycle and was used to determine the amount of crack growth. It appears that the GIc values (fracture toughness) may be lowered slightly as the moisture content of the atmosphere increases. It was also found that the water vapor alone assists slow crack growth, but that water vapor combined with repeated loading has a greater effect. The effect can be easily seen by looking at the ratio σn′σys where σn′ is the stress level sufficient to cause either slow crack growth or failure in less than 100 cycles. An interesting result is that if a specimen was cycled in the 100 percent relative humidity condition and with a natural crack present, the σn′σys ratio was found to be about 0.30 while it was about 0.55 if no moisture was present.