In mechanical engineering design it is common for multiple stress concentration areas to coexist adjacently, where the existing analytical theory cannot be used to assess the effect. The effect of adjacent step and keyway on a DIN CK 45 shaft has been investigated in the present paper. For this purpose, a DIN CK45 steel shaft specimen with a standardized keyway and a diameter-step has been tested under rotating bending in order to investigate the interaction of two distinct stress concentration areas. All fragment specimens have metallographically examined. Analytical and numerical methods have been used to support the conclusions.