Low dose corticosteroids are effective in suppressing synovitis in rheumatoid arthritis (RA), but there remains concern about their side effects, particularly osteoporosis. To examine the effects of low dose corticosteroids on bone loss in RA bone mineral density (BMD) was measured in the lumbar spine and hip for up to two years in 15 patients treated with these agents (mean dose prednis(ol)one 6.6 mg/day). 15 patients not receiving them, and 15 age matched controls. The initial BMD at both skeletal sites was significantly reduced in both patient groups compared with controls. The mean change in bone density was 0.2, 0.1, and -0.1% a year in the spine and -2.0, -1.9, and -1.0% a year in the hip respectively for the three groups. These rates of bone loss were not significantly different between groups at either site. These findings suggest that low dose corticosteroid treatment in RA is not associated with an increased risk of osteoporosis.