Abstract: A hip-joint prosthesis has a ball head shaped to fit in an acetabulum, an elongated shaft adapted to fit in and between two opposite sides of a medullary cavity of a femur, and a stem between and connecting the head to one end of the shaft. This shaft is curved in a plane and has a concave longitudinal edge and a convex longitudinal edge. A laterally projecting shoulder between the stem and the shank is engageable against a sawed-off end of a femur in whose medullary cavity the shaft is engaged. The shaft is formed on its concave edge at its two ends with inner and outer engagement surfaces of planar shape. Between these engagement surfaces on the convex side there is formed an intermediate engagement surface. When mounted in a femur the inner and outer engagement surfaces engage against one side of the medullary cavity and the intermediate engagement surface against the other for firm mounting of the shaft in the femur.