Femoral bone mass has been measured in 42 patients with rheumatoid arthritis and in 27 with osteoarthrosis. Measurements were made by scanning the leg with a photon beam from 241Am. The results have been compared with normal subjects and correlated with clinical, biochemical and radiological parameters of the arthritic patients. In general, femoral bone mass was lower than normal in patients with arthritic changes. Reduced physical activity rather than the arthritis itself seemed to be the cause of the low bone mass. In rheumatoid arthritis the bone mass appeared to be an accurate representation of the clinical assessment of the disease.