Rheumatoid arthritis may be associated with generalised as well as periarticular osteoporosis. To assess the extent of bone loss and the influence of corticosteroid treatment total body calcium was measured by in-vivo neutron activation analysis in 63 patients with rheumatoid arthritis treated with non-steroidal anti-inflammatory drugs alone and 31 treated with additional low-dose corticosteroids. The results were compared with those in 40 normal controls matched for age, sex, and menopausal state. There were significant reductions in mean total body calcium in the group treated with non-steroidal anti-inflammatory drugs (5.3% in men; 6.8% in women) and greater reductions in the corticosteroid-treated patients (11.5% in men, 15.5% in women). The reduction was correlated with disease duration and activity in the patients treated with non-steroid anti-inflammatory drugs alone. Measured total body calcium was significantly less than the values predicted when this relation was used in the corticosteroid-treated patients. The data suggest that increased bone loss in patients with rheumatoid arthritis treated with corticosteroids is attributable to drug treatment rather than disease activity. Many patients with rheumatoid arthritis treated with low-dosage corticosteroids and some postmenopausal women with the disease are likely to be at risk from the complications of osteoporosis.