We show two case reports to discuss the problem of fatigue fractures of non-cemented porous coated total hip prosthesis. Fatigue fractures occur in the region of highest tensile stresses. Because of the surface irregularities of the porous coating, tensile stresses in the surface of the stems are increased depending on the configuration of the pore. A thick porous coating also decreases the cross section of the stem. In addition the manufacturing process, the application of a porous layer can weaken the stem. Finally, cementless prostheses are implanted especially in young active patients, who exert high forces on their hip. For cementless porous coated hip prostheses, we need a standardized manufacturing process and test of the fatigue strength. In addition, there should be a minimum diameter of the core of the stem in order to avoid fatigue fractures. Key words: porous coated total hip prosthesis, fatigue fracture of the stem.