Background Vitamin D has been found to have a role in the function of the immune system.There have been a lot of studies investigating a relation between vitamin D and disease activity in ankylosing spondylitis (AS).However,there have not been any studies arranging AS in groups according to vitamin D levels and determining any differences among these patients in terms of disease activity,functional status,quality of life,and other clinical parameters.The aim of this study is to compare 25-hydroxy-vitamin D3 (25(OH)D3) levels in AS patients with those in normal healthy subjects and to determine the relationship between 25(OH)D3 levels and AS disease activity,functional status,and quality of life.Methods Ninety-nine consecutive patients and 42 healthy volunteers were included in this study.After a comparison between the patient group and the control group,the patient group was divided into normal,insufficient and deficient subgroups according to the plasma 25(OH)D3 levels for another comparison.Results The differences in the 25(OH)D3 level between the patient and the control groups were statistically insignificant.The number of AS patients whose 25(OH)D3 levels were classified as normal,insufficient,and deficient were 34,29,and 36,respectively.Erythrocyte sedimentation rate (ESR),C-reactive protein (CRP),and Bath AS Disease Activity Index (BASDAI) scores were higher in the low (including insufficient and deficient) 25(OH)D3 level subgroups (P＜0.05).The Bath AS Functional Index (BASFI) and AS Quality of Life (ASQoL) scores were significantly different between the normal and the deficient subgroups (P＜0.05).Pain,BASDAI,ESR,and CRP were inversely correlated to the 25(OH)D3 levels (P＜0.05).Conclusions The plasma 25(OH)D3 levels may decrease in AS patients and this may negatively affect disease activity,functional status and quality of life.