The lymphocyte-osteoclast interaction has recently been described. The aim of this study was to investigate the possible relationship between ankylosing spondylitis (AS) and bone metabolism. Bone metabolism was evaluated in the blood of 49 patients with AS by means of biochemical markers and bone mineral density (BMD) with a Lunar device. Bone formation markers, bone specific alkaline phosphatase (BALP), osteocalcin (BGP), bone resorption markers, pyridinoline (Pyd), deoxypyridinoline (Dpyd) and lymphocyte surface markers (CD3, CD19, CD4, CD8, CD16+56) were analysed with ELISA and flow-cytometry methods. The patients had significantly lower femoral neck and trochanter BMD than the controls. Dpyd concentrations were negatively correlated to CD3+% and CD3-/CD16+56% cells. Neither mineral nor hormone levels were significantly correlated with absolute T scores of BMD of the hip sites. BALP and BGP were negatively correlated to BMD when expressed as T scores. We conclude that AS is related to accelerated osteoclastic activity. Many lymphokines and growth factors produced by lymphocytes can influence osteoclastogenesis and probably play a role in rheumatologic/inflammatory disorders.