Rotating cantileverfatigue tests were performed on specimens extracted longitudinally from the cortices of human femora. The usual inverse relationship between stress amplitude and fatigue life was found; the median life was 1.3×10⁴ cycles at a stress amplitude of 12,200 lbf/in² (84.1 MN/m²) and 4.6×10⁶ cycles at 6750 lbf/in² (46.6 MN/m²). Arguments are advanced which suggest that these results are compatible with what is known of the stresses applied in life and the clinical occurrence of fatigue fractures.