The aim of this work was to determine bone mineral density (BMD) in a group of patients with ankylosing spondylitis (AS) and to study alterations in bone remodeling in these patients. Eighteen patients (16 males and two females) with AS, mean age 44.7, range 21-75, and 18 age- and sex-matched healthy controls were studied. BMD was evaluated by dual energy X-ray absorptiometry. The following biochemical markers of bone remodeling were studied: formation - serum amino and carboxyterminal propeptides of procollagen I (PINP and PICP); resorption - urinary total and free deoxypyridinoline and pyridinoline (TDpyr, FDpyr, TPyr and FPyr), crosslinked aminoterminal telopeptides of collagen I (NTX), carboxyterminal telopeptide of collagen I (CTX) and serum bone sialoprotein (BSP). Receiver operating characteristic (ROC) curves of markers were also performed. We found a decrease of bone mass and an increase in TPyr, FPyr, TDpyr, FDpyr, NTX and BSP in AS, but no significant differences were found in PICP, PINP and CTX. FDpyr, FPyr and TPyr showed the highest discrimination between patients and controls according to the results of the ROC curves. TPyr/TDpyr was higher in AS than in controls. We found osteopenia, with a normal formation and a significant increase in bone resorption in AS. FDpyr, FPyr and TPyr seem to present the best sensitivity for the study of alterations of bone resorption in this pathology, although NTX, TDpyr and BSP also show significant differences. The elevation in the ratio TPyr/TDpyr in AS compared to controls indicates that in AS there is a type I-collagen degradation in tissues different from bone.