Background Radiographic damage is one of the core outcomes recommended by the ASAS for follow-up of patients with AS. So far, the evolution of radiographic damage over a long period has not been described. Objectives To describe the evolution of radiographic abnormalities in patients with AS. Methods The modified Stoke AS Spine Score (mSASSS) was calculated using 2-yearly radiographs of patients followed for 12 years in the Outcome in AS International Study (OASIS). Two readers (R1 and R2) independently scored the x-rays and scores were averaged. Status and progression scores (2-year and 12-year-progression) were computed for patients with at least one 2-year interval available (n=186) and for those also with an mSASSS at 12-years (n=68). New syndesmophytes at vertebral corners (VCs) at risk (i.e. without a previous syndesmophyte or bridge) were computed. Radiographic progression over time was investigated using generalized estimating equations. Relevant interactions with time were explored. Time was modeled in different forms (linear and non-linear). Results 809 radiographs of 186 patients (70% males, mean(SD) age 43(12) years, mean disease duration 11.0(8.8) years and 83% HLA-B27 positive) were included. The mean(SD) mSASSS at baseline was 11.6(16.2) [11.2(15.7) in patients with mSASSS at 12-years]. The mean(SD) 2-year progression (in 520 intervals) was 2.0(3.5) [2.2(3.9) for the subset of 12-year-completers]. Over the 12 years, the mean(SD) progression was 11.7(11.5). A new syndesmophyte was assessed in 38% (R1) and 39% (R2) of all 2-year intervals and, throughout follow-up, in 55% (R1) and 63% (R2) of the patients with at least one VC “at risk”. In 24% of the patients (39% of the 2-year intervals) there was no progression in mSASSS. A progression≥5 mSASSS units occurred in 22% of the patients (12% of the intervals). At the group level, a linear time course model fitted the observed data the best. Time was positively associated with radiographic progression, with an increase of 0.98 mSASSS/year. Radiographic progression occurred faster in males, in HLA-B27 positive patients and in patients with a baseline mSASSS≥10. HLA-B27 positive male (but not female) patients had a significantly higher progression than HLA-B27 negative males. Progression was independent of disease- and symptom duration. Conclusions Long-term radiographic progression in AS is more severe in HLA-B27 positive males. About 60% of all patients have at least one new syndesmophyte over a period of between 2 and 12 years. Radiographic progression is dependent on time and, at the group level, linear. Disclosure of Interest None Declared