To study the prevalence of osteoporosis (OP) and osteopenia in ankylosing spondylitis (AS) and to investigate the relationship between symptomatic and structural severity, the indices of bone turnover, and body composition. Eighty patients with AS were enrolled prospectively: 52 men (65%) and 28 women, mean age 36.7 years +/- 11.5 (range 18-67); they were studied clinically, radiologically, and by dual energy x-ray absorptiometry. Sixty-three underwent biological assessment of bone turnover markers. OP and osteopenia as defined by the World Health Organization (T score < -2.5 SD and between -1 and -2.5 SD, respectively) were observed in 15 (18.7%) and 25 patients (31.2%) at the lumbar spine and in 11 (13.7%) and 33 patients (41.2%) at the femoral neck, respectively. Patients with OP had a lower body mass index (BMI) and fat mass percentage. There was a trend to a lower disease duration in patients with OP at the spine than in healthy subjects. Bone resorption markers (urinary D-pyridinoline or C-telopeptide concentrations) were increased in 34 patients (53.9%). Bone turnover markers were positively correlated with C-reactive protein concentration and Larsen radiological hip score; they were negatively correlated with Schober index and fat mass percentage. (1) OP is frequent in AS and can be observed in early stages of the disease. (2) Patients with AS are more susceptible to develop OP when they have low BMI, low fat mass percentage, and active and severe disease. OP was observed in parallel with increased bone resorption.