The inclusion criteria was patients who aged 18 or older with ankylosing spondylitis, fulfilled the 1984 modified New York criteria for AS. Inclusion criteria enriched the AS patients with clinical remission, including the following definition: 1. Administration of etanercept 50 mg for 6-week period at least; 2. Acquisition of Assessment of SpondyloArthritis International Society criteria 20(ASAS20) response at the end of the treatment. We excluded patients who have developed to complete spinal fusion. We also excluded patients with kidney disease induced by other conditions; pregnancy; suckle; accompany other chronic diseases; various infections in acute stage; and other infectious diseases. At the end of the trial, patients who fulfilled the inclusions would stop etanercept treatment. Cotherapy with disease modifying anti-rheumatic drugs or non-steroidal anti-inflammatory drugs could be continued if maintained at a stable dose;Patients were followed up from the time of etanercept withdrawal per 6 weeks for 3 years by telephone. If symptoms suggestive of relapse or other problems occurred, patients were invited to come back to the center. Relapse after etanercept withdrawal was defined as an increase Bath Spondylitis Disease Activity Index(BASDAI)15 score goes back to 80 percentages of it at the beginning of the trial16. The following data were collected: demographic and disease characteristics, therapeutic modification, clinical values (BASFI, Bath Ankylosing Spondylitis Global Score (BAS-G)), Ankylosing Spondylitis Disease Activity Score (ASDAS)1718) and biologic values at baseline of the trial and the time of relapse. Adverse events and other safety measures were also collected.

Patients were followed up 6 weeks by telephone. The sample size was at least 30 people. If symptoms suggestive of relapse or other problems occurred, patients were invited to come back to the center. Relapse after etanercept withdrawal was defined as an increase Bath Spondylitis Disease Activity Index(BASDAI) score goes back to 80 percentages of it at the beginning of the trial16. The following data were collected: demographic and disease characteristics, therapeutic modification, clinical values (BASFI, Bath Ankylosing Spondylitis Global Score (BAS-G)), Ankylosing Spondylitis Disease Activity Score (ASDAS) and biologic values at baseline of the trial and the time of relapse. Adverse events and other safety measures were also collected.

Demographic and baseline disease characteristics were summarized with descriptive statistics and analysed with one-way ANOVA for continuous for continuous variables and χ2 tests for categorical variables. The Kaplan-Meier method was used to estimate the time-to-relapse rate after etanercept withdrawal. Time-to-relapse curves were compared between the group of patients received 12-week and 6-week treatment of etanercept through log-rank test. The influence of the following variables including age，duration of disease, onset age, BASDAI, ASDAS-CRP, C reaction protein(CRP), and erythrocyte sedimentation rate(ESR) was examined using the Cox proportional hazards model to evaluate at etanercept withdrawal on time-to-relapse. Every continuous variable was divided into 3 categories at approximately the 33% and 67% at first19. If the relative relapse rates were not significantly different in 2 contiguous categories, they were gathered together. If no clear difference was observed in 3 categories, the median was used as a cut-off point. Normal value such as 6 mg/L for CRP level were tested. The proportional hazards model was used to study the effect of each factor on time-to-relapse and identify the independent prognostic factors. Relapse rates are presented as estimate with standard error (SE), follow-up times as median (interquartile range), and hazard ratio as estimate with 95% confidence interval.

All analyses were performed using SPSS software v16.0 (SPSS, Inc, Chicago, IL).

Inclusion Criteria:

• Clinical diagnosis of ankylosing spondylitis(1984 modified New York criteria).
• Administration of etanercept 50 mg for 6-week or 12-week.
• Acquisition of ASAS20 response at the end of the treatment.

Exclusion Criteria:

• Complete spinal fusion.
• Kidney disease induced by other conditions; pregnancy; suckle; accompany other chronic diseases; various infections in acute stage; and other infectious diseases.
• Refused to discontinue etanercept treatment.

The data will be published on the Journals.