Aim: A debonding in the stem-cement interface is often the þrst event leading to clinical failure of hip prostheses. We have tested the stability of a pre cement mantled stem were the concept of composite beam stems is taken to a new level. Methods: Twenty four patients with a mean age of 64 years (48–73), a weight of 72 kg and allocated to total hip arthroplasty due to OA were operated. All received a Deþnition stem (a straight CoCr stem proximally coated with a PMMA mantle). All poly cups, Palacos cement and 4th generation cementing technique was used in all. Tantalum markers were inserted in femur and the stem and migration measured with RSA at 2, 12 and 24 months. Radiographs and Harris hip score was obtained post op and at 2 years. Results: The subsidence of stems in relation to femur was 0.00 mm (SEM 0.02) and the cement mantle subsided 0.2 mm (SEM 0.02) in relation to femur. The femoral head rotated backward 0.02 mm.(SEM 0.07) Post op radiolucent lines of 1 mm thickness or more were present to a mean of 0.3% (0–3%) in the cement-bone interface at 2 years. Harris hip score increased from a mean of 46 (23–68) pre op to 93 (57–100) at 2 years. Conclusion: Deþnition is the þrst stem investigated with RSA were no migration at all could be recorded the þrst 2 years. Togeher with a stable cement mantle and no radiolucent lines it so far indicates a good long term performance of the Deþnition stem.