To determine the correlation between the bone mineral density (BMD) and spinal mobility and chest expansion index in patients with ankylosing spondylitis. Eighty patients with confirmed diagnosis of ankylosing spondylitis were included in this study. In all of them physical examination was performed including assessment of spinal mobility and chest expansion index. Bone mineral density of the lumbar spine (L1-L4, anteroposterior view) and at the left hip was measured by dual X-ray absorptiometry (DXA) in standard manner According to the WHO classification of osteoporosis, patients were classified in three groups (normal, osteopenic or osteoporotic) depending on the osteoporotic status in lumbar spine, hip and femoral neck region. Eighty patients (46 men and 34 women; age 25-73 years) were included. Mean BMD for lumbar spine was 1.104 +/- 1.043 (T score: 0.67 +/- 2.15) and for total hip was 1.057 +/- 0.899 (T score: -0.28 +/- 2.34). Significant difference in the mobility of thoracic spine was observed in patients in regard to the WHO classification of osteoporosis in lumbar and femoral region (p = 0.031, Oneway Anova for osteoporosis of lumbar region; p = 0.022, Oneway Anova for osteoporosis of total hip region). Mean value for the chest expansion index was 3.07 +/- 1.66 cm. Chest expansion index was significantly reduced in patients having osteoporosis in lumbar and total hip region (p = 0.015, Oneway Anova for osteoporosis of lumbar region; p = 0.038, Oneway Anova for osteoporosis of total hip region). The observation that reduced mobility of thoracic and lumbar spine and chest expansion index occured in patients with low BMD in lumbar and total hip region suggest that osteoporosis should be monitored more frequently in patients with AS.