The aim of this study was to assess the vitamin D status in patients with ankylosing spondylitis (AS), and to investigate the relation between vitamin D levels, bone mineral density (BMD) and disease activity in men with ankylosing spondylitis. Seventy patients with AS and 140 healthy individuals were included in the study. BMD of femur and lumbar spine was measured by DXA. Serum 25OH vitamin D, parathormone, serum calcium, Creactive protein levels of all participants were also measured. The disease activity was evaluated by Bath Ankylosing Spondylitis Disease Activity Index (BASDAI), and functional status by Bath Ankylosing Spondylitis Functional Index (BASFI). The mean 25(OH)D level was 17.5+9.7 ng/mL in AS patients and 21.9+7.7 ng/mL in controls (P<0.001). In comparison with the control group, AS patients showed significantly higher CRP, and a significant reduction of vitamin D. In AS group, 62 patients (88.6%) had Vitamin D deficiency, and 35 patients (50%) were osteoporotic. Vitamin D was negatively correlated to BASDAI without any changes after adjustment for age, duration of disease, sunlight exposure, and total taking steroids (r=-0.32, P<0.001).We found a high incidence of vitamin D deficiency in our patients. Our study suggests that vitamin D deficiency in male AS may indirectly lead to osteoporosis by causing an increase in the inflammatory activity. Monitoring vitamin D levels would be useful in order to determine the patients under osteoporosis risk.