Torsional instability of femoral components has not received much attention, and is difficult to detect in conventional radiographs. To test this we designed a system to apply a load in an anteroposterior direction to the head of a femoral component, implanted into a cadaveric femur. Rotation within the bone was measured, using a purpose built transducer, with and without preservation of the neck, with and without cement, and with longitudinal ridges but no cement. The results show that torsional instability may be a problem in uncemented replacement. Preservation of the femoral neck and the use of a ridged prosthesis increases resistance to rotation. Rotational movements occurring in vivo during such activities as climbing stairs and rising from the seated position may contribute to mechanical loosening.