Environmentally enhanced fatigue of medium and high strength steels is generally believed to involve a hydrogen embrittlement mechanism. As part of a wider study, smooth samples of HSLA steel in two grain sizes were subjected to cycling in hydrogen and nitrogen at a load ratio, R = -1 at 20 Hz. From curves of stress/strain amplitude vs number of cycles to failure, it is readily apparent that a larger grain size results in a more marked decrease in fatigue resistance in 1 atm. of hydrogen. This behavior is attributed to grain boundaries acting as trap sites for hydrogen with ensuing grain boundary embrittlement. This assertion is supported by fractographic examination which reveals that crack initiation and propagation are predominantly intergranular in hydrogen. Initiation in the absence of hydrogen is strongly associated with crack nucleation along slip bands with subsequent transgranular propagation. For life prediction, with or without environmental influences, initiation can be assumed to be the most difficult regime to predict, given that long crack growth can be represented by LEFM curves and the small crack growth regime typically involves relatively few cycles when compared with the other regimes. A model is proposed for initiation in the absence of an embrittling species and the concept of hydrogen induced fatigue thresholds is presented.