BACKGROUND: Osteoporosis and vertebral factures are well recognized features in patients with ankylosing spondylitis (AS). The aim of this study was to investigate the prevalence and risk factors of osteoporosis and vertebral fractures in patients with AS. METHODS: Fifty-nine AS patients and 40 healthy controls were enrolled. Bone mineral density (BMD) was measured using dual-energy X-ray absorptiometry (DEXA) at posterior-anterior (PA) lumbar, lateral lumbar and hip regions. Thoracic and lumbar X-rays were obtained for morphometric measurements. Clinical, biological and radiological statuses were evaluated with Bath Ankylosing Spondylitis Disease Activity Index (BASDAI), Bath Ankylosing Spondylitis Metrology Index (BASMI), Bath Ankylosing Spondylitis Functional Index (BASFI), Bath Ankylosing Spondylitis Radiology Index-total (BASRI-t), erythrocyte sedimentation rate (ESR) and the C-reactive protein levels. RESULTS: Osteoporosis was present in 32% of patients and 5% of controls according to lateral vertebral BMD measurements. Fracture was present in 31% of patients. The effect of some clinical and laboratory parameters on BMD status and vertebral fractures was analyzed in the patient group. Osteoporosis in lateral lumbar DEXA was associated with higher BASMI, BASFI, BASRI-t scores and ESR level. Low hip BMD was associated with low BMI and high BASFI and BASRI-t scores. Vertebral fractures were associated with advanced age, longer disease duration, longer duration since diagnosis, higher BASMI and BASRI-t scores, higher ESR level, reduced femoral and lateral lumbar BMD. Logistic regression analysis revealed that only BASRI-t score was significantly associated with low lateral spinal BMD and BMI and BASFI score were independently associated with low hip BMD. The presence of compression fractures was independently associated with BASRI-t score and low lateral lumbar BMD. CONCLUSIONS: Osteoporosis and vertebral fractures in AS seem to be related to the extent of radiological involvement. A low lateral lumbar BMD is an important risk factor for vertebral fractures.