A system for roentgen stereophotogrammetry has been used in Lund, Sweden, since August 1972 and by August 1982 ten thousand stereo films have been evaluated using this method. Patient investigations started March 1973 and of the more than 500 patients investigated 300 have been referred by orthopedic surgeons. The skeletal parts investigated had been permanently marked with tantalum balls 0.5 or 0.8 mm in diameter. The causes for the investigation have been bone growth disorders in the Lower extremity (134 patients), spinal fusions (35 patients), high tibial osteotomies for gonarthrosis (21 patients) and endoprosthetic replacement of the knee (68 patients) or hip (42 patients) joint. By the roentgen stereophotogrammetric analysis (RSA), information on for instance the development of angular deformities and staple loosening in the leg, the healing process of the intended fusion in the spine or at the knee, and on the migration and stability of prosthetic components have been obtained. RSA is of value both for prognostic and followup purposes, and is versatile enough in proper setting to be used as a routine clinical investigation.