Estimation of the 3D pose of bones in 2D images plays an important role in computer-assisted diagnosis and surgery. Existing work has focused on registering a 3D model of the bone to the images of the same bone. This approach incurs a high health care cost on the patients, and are prescribed only for certain procedures. Such computer assistance is not routinely available to diagnostic and surgical procedures that involve only x-ray images, limiting the precision of the procedures. This paper proposes a method for recovering 3D pose by registering a generic 3D model of a bone to single x-ray images of different patients. It can perform 3D-2D registration with unknown correspondence automatically. Comprehensive test results show that the method is accurate and robust in recovering the pose of femurs in the x-ray images of different patients.