The quality of the subchondral bone in total knee arthroplasty (TKA) may be important for the outcome. We correlated a histomorphologic analysis of the prosthetic bone bed to micromotion as analyzed by roentgen stereophotogrammetric analysis (RSA). Twenty-three knees, both osteo- and rheumatoid arthritic were studied. The mean migration after 1 year was 1 mm (0.21-2.68), with no correlation to the bone variables or diagnosis. One patient underwent revision after 3 years because of pain. One patient underwent revision after 15 years because of aseptic loosening. This patient appeared stable according to changes in maximum total point motion (MTPM), but the prosthesis migrated continuously according to segment motion. Prosthetic migration increased after 10 years, zones started to evolve, and the patient became symptomatic after 13 years. A biopsy of the bone bed was taken in all patients. A larger percentage of trabecular bone and a larger relative area of unmineralized osteoid surface were found in patients with rheumatoid arthritis compared with those with osteoarthritis. The latter finding is probably a result of increased bone turnover. In conclusion, no support currently exists for the idea that preoperative bone quality, evaluated as bone histomorphometry, has any impact on prosthetic micromotion in knee prostheses.