The influence of a dry hydrogen environment on near-threshold crack growth propagation rates of fatigue cracks in a low-alloy spheroidized steel was investigated. for separation of environmental and mechanically induced effects, fatigue tests in an ultra-high vacuum (UHV) environment were taken as reference. On a macroscopic scale, we found a significant acceleration of the propagation rates of cracks exposed to a dry hydrogen atmosphere compared to tests in an inert environment. The electron-microscopic characterization of the microstructure in the vicinity of cracks revealed that the acceleration of fatigue propagation by hydrogen can be rationalized by a hydrogen-involved fatigue damage mechanism. Furthermore, it was concluded that hydrogen enhances the dislocation mobility. This effect aids in the formation of dislocation cellular structure in the cyclic plastic zone. The mode of fracture itself is ductile transgranular and appears to be unchanged by absorption of hydrogen at the crack tip.