OBJECTIVES: Ankylosing spondylitis (AS) is associated with poor quality of life and higher health care cost. This study aimed to assess the inpatient costs of AS and to characterize predictors of these costs. METHODS: A total of 560 AS patients were randomly selected by stratified two-stage sampling from the China Basic Health Insurance database in 2009. All information of patient demographic characters, clinical and costs were collected for the analysis. We used generalized estimating equations to examine potential predictors of the costs. RESULTS: Among 560 AS patients (mean age = 55.7 years;57% female), The mean hospital length-of-stay was 15.2 days for AS patients with basic medical insurance for urban employees and 9.8 days for those with basic medical insurance for urban residents(P<0.001). The average inpatient cost was RMB 8173.3 yuan (median:4368.7, IQR:2836.1-7399.7) and the average medical service cost accounts for 68.0% of the total cost (mean: 5560.5; median:2376.0, IQR:1395.0-3882.7). The multiple linear regressions showed that the patients from tertiary hospitals had 84.7% higher costs than those from primary hospitals (P <0.001) , patients from the east area have 30.7% higher costs than those from the west area ( P <0.01), and patients from municipalities had 72.5% higher costs than those from county-level cities( P <0.05). CONCLUSIONS: Patients with AS is associated with high hospital costs. Costs are now driven predominantly by the cost of medical service, and sociodemographic characteristics such as regions and levels of hospitals also play an important role in determination of costs.