Manganese sulfides (MnS) are nonmetallic, ductile inclusions with high melting temperature (1610 °C) which improve the machinability and retard the grain growth in steels, in addition to contributing to avoid cracking during hot working. In this paper, the effect of manganese sulfides on the fatigue life of forged steel is discussed. As the initial state a wrought product of rolled steel with a high sulfur content of 0.073% is used. Orthogonal to the direction of rolling, a cylinder is extracted and forged in the axial direction, decreasing the height by a factor corresponding to an upset (logarithmic) strain of -1.35 at a temperature of 1200°C. Due to this upset forging process, the MnS are compressed to thin and long particles. In addition to the upset forged block material, samples from the lateral position with lower content of MnS and from the center position of the wrought material are characterized by microsections and metallographic analysis to evaluate the shape and content of MnS. Small-scale specimens extracted from the two previously defined positions within the wrought material and from the center of the forged material are subjected to force-controlled fatigue tests with a stress ratio of R=-1 at a frequency of 150 1/s using a piezo actuator. The fracture surfaces are evaluated with respect to shape and content of MnS. The fatigue life of the forged material, presented by Wöhler curves, is considerably reduced at high levels of the nominal stress amplitude compared to the wrought material. Moreover, it is evident that the presence of a high content of thinner MnS reduces the scatter band of Wöhler curves. To isolate the effect of the MnS content, fatigue tests on specimens from the center and the outer side of the wrought material are compared.