To assess bone mass in ankylosing spondylitis (AS) we have measured total body calcium, bone mineral content of the lumbar spine and metacarpal indices in groups of patients with AS. Mean total body calcium was reduced by 5.3% (p less than 0.05) in 20 patients compared with controls. The mean annual loss of bone, assessed over an 18 month period in 17 patients, was 2.9% (p less than 0.001). Compared to controls, bone mineral content was increased by 28% (p less than 0.05) in 8 male patients while metacarpal indices were normal in 18 male and female patients. The results of total body calcium measurements give support to the hypothesis of a minor increase in bone turnover in AS. The increased bone mineral content in the male patients may relate to syndesmophyte formation.