Two cases have been presented in which the nail was so firmly fixed in the cortex of the femoral shaft that, as the neck shortened, the nail penetrated the acetabulum. In the usual case, with similar femoralneck neck shortening, the nail has been extruded. In the first of these two cases it is difficult to explain why the nail was so firmly fixed. In the second case fixation was probably due to an overgrowth of callus over the nail head. These two cases, and the observation that there is measurable shortening of the femoral neck in a large number of patients, point to the danger of mechanical fixation of the nail head to the femoral shaft. Such fixation has been advocated by several surgeons using various methods. Even when mechanical fixation has not been done, as in these two cases, the possibility of perforation of the femoral head should be considered whenever a previously satisfactory nailing becomes painful on motion.