Psoriatic arthritis is thought to be associated with periarticular osteoporosis while rheumatoid arthritis may be associated with generalised as well as periarticular bone loss. To assess the extent of total and peripheral bone loss in these two diseases, total body calcium was measured by in vivo neutron activation analysis and peripheral bone mass was assessed by metacarpal indices in age-matched patients with psoriatic arthritis and rheumatoid arthritis treated with nonsteroidal anti-inflammatory drugs alone. In comparison with age and sex-matched normal controls, total and peripheral bone mass was normal in psoriatic arthritis. There were significant reductions in total (6.2% in men; 7.9% in women) and peripheral (10.9% in men; 12.8% in women) bone mass in patients with rheumatoid arthritis compared with controls. Peripheral bone mass was significantly correlated with the degree of radiographic damage in male and female patients with rheumatoid arthritis. The mean annual loss of total body calcium was insignificant in psoriatic arthritis (0.6% in men; 1.9% in women) but markedly greater in rheumatoid arthritis (4.4% in men; 2.7% in women). The data suggested that total and peripheral bone loss is greater in rheumatoid arthritis than psoriatic arthritis. Substantial reductions in peripheral bone mass in patients with rheumatoid arthritis not receiving corticosteroids may account in part for the small reductions in total bone mass.