Dual x-ray absorptiometry (DXA) has become the most used and proven to be a precise method to investigate changes in bone mass density (BMD). Research with DXA has shown the pattern of bone remodelling around an implant. Results from implant studies are often presented as changes in BMD as a function of time. The baseline and reference value for such calculations is the first measurement after the operation. The most pronounced bone loss after implantation of hip prosthesis usually comes within 3 months. The baseline measurement is performed at different time from study to study. If there is a rapid bone loss the first weeks after an operation, this will influence the reference value and then the results. To evaluate DXA as a method we decided to set up a study with 3 DXA measurements within the first 2 weeks.