Objective The goal of this study was to investigate the correlation between the level of serum vitamin D and bone metabolism and disease activity in ankylosing spondylitis(AS) patients,and thus to explore the role of vitamin D in bone metabolism in AS patients.Methods The serum levels of BALP,TRACP-5b,25-(OH)D3 and blood lymphocytes VDR of 80 AS patients were measured by enzyme-linked immunosorbent assay (ELISA) and compared with those of the control group.Bone mineral density (BMD)was measured by dual-energy X-ray absorptiometry (DEXA).The ESR and CRP level of AS patients were also measured.The correlation between those parameters was analyzed and evaluated.Patients were divided into normal,insufficient and deficient subgroups according to the serum 25-(OH)D3 levels for further comparison.Indepondent saimple t test,t'test andx2 test were used for statistical analysis.Results The 25-(OH)D3 of AS patients [(11.9±2.7) μg/L] was significantly lower than that of the control groups [(22.3±7.9) μg/L] (P＜0.05),while the serum levels of BALP [(3.9±2.7) μg/L] and TRACP-5b [(46±25) ng/L] of AS patients were significantly higher than those of the control group [(2.4±1.0) μg/L] (P＜0.05).According to linear correlation analysis,25-(OH)D3 was negatively correlated with CRP (r=0.324,P=0.003).The ESR,BALP,TRACP-5b in the deficient subgroup were higher than those in the normal and insufficient subgroups(P＜0.05).Conclusion The plasma 25-(OH)D3 may decrease in AS patients,and this may activate bone metabolism,results in increased morbidity of osteoporo-sis,and negatively affect disease activity.