A femoral sizing guide comprises a body (6), a superstructure (40) and a stylus (53). The body (6) is arranged to rest against a resected femoral surface and has first and second feet (12) arranged to extend underneath respective posterior condyles and rest against posterior condylar surfaces of the femur. The superstructure (40) is coupled to the body (6) and is arranged to slide parallel to the resected femoral surface towards and away from the feet (12). The stylus (53) is coupled to the superstructure (40) and arranged such that when the body (6) rests against the resected femoral surface a tip (60) of the stylus (53) extends over the femur such that sliding the superstructure (40) towards the feet (12) causes the stylus tip (60) to contact the anterior cortex of the femur. The superstructure (40) further comprises a first guide hole (52) defining a first alignment axis extending into the resected femoral surface at a predetermined distance from the level of the stylus tip (60) in the plane of the resected femoral surface. The body (6) defines a second guide hole (22) defining a second alignment axis extending into the resected femoral surface at a predetermined distance from the feet (12), the distance between the first and second guide holes (52, 22) varying as the superstructure (40) slides relative to the body (6).
