To assess mechanisms that cause generalized osteoporosis in rheumatoid arthritis (RA), we measured bone mineral density (BMD) by dual photon absorptiometry in the lumbar spine and femoral neck of 111 patients with RA. BMD was significantly reduced at both sites in these patients. Physical activity correlated significantly with BMD in patients with RA, and was found, by multiple regression analysis, to be a significant predictor of femoral bone density in female patients. Multiparity exerted a protective effect on lumbar bone density. Prednisolone (mean dosage 8 mg/day) was not associated with significantly increased bone loss in women, whereas higher dosages in men (mean 10.3 mg/day) were associated with increased lumbar bone loss. Reduced physical activity leading to a form of disuse osteoporosis appears to be an important factor in axial bone loss in RA.