The cortical bone mineral density (BMD), bone volume, bone mass and muscle volume of the thigh, and the BMD of the distal femur and proximal tibia were measured quantified by quantitative computed tomography (QCT) after an operation for a displaced femoral neck fracture. Twenty patients were randomized to osteosynthesis or total hip arthroplasty (THA). Both legs were scanned after 18 months, and the operated side was compared with the healthy side. Clinical assessment was performed with a Harris hip score. A reference group of 9 patients, who had undergone THA because of arthrosis, was chosen. In the fracture patients, we found a 9% decrease in bone mass and muscle volume of the middle femur. The BMD of the distal femur and proximal tibia showed a more marked osteopenia. There was no difference in these parameters between the two groups. In the reference group of operated arthrosis patients, we did not find any differences between sides postoperatively. After the operation, the fracture patients had a lower Harris score than the arthrosis patients, and this was most pronounced among those who had undergone osteosynthesis. The finding of a marked osteopenia after a femoral neck fracture, irrespective of treatment, but no bone loss after THA because of arthrosis, implies that patients with a femoral neck fracture are more sensitive to osteopenia, and that the bone loss is not proportional to the operative trauma.