The flat bending fatigue strength of a variety of carbon-free iron base sintered materials has been investigated varying density, sintering temperature and particularly the morphology of the base powder. All effects that lead to round pores improve the fatigue performance. The porosity of sintered iron and steel is characterised by a large majority of tiny spherical pores and a few big pores of high irregularity. It is this small fraction of large and irregular pores with their pronounced internal stress concentration which determines the fatigue strength of sintered porous materials, next to overall density and alloying effects.