GLAS cohort is an ongoing prospective longitudinal observational cohort study. This cohort is embedded in daily clinical practice and patients are included in the northern part of the Netherlands. Since November 2004, consecutive ankylosing spondylitis (AS) outpatients who started TNF-α blocking therapy because of active disease at the University Medical Center Groningen (UMCG) and the Medical Center Leeuwarden (MCL) were included in this cohort. In 2009, this inclusion was extended to all consecutive axial spondyloarthritis outpatients, irrespective of treatment regimen. Patients treated with TNF-α blocking therapy are evaluated at baseline, after 3 and 6 months, and then every 6 to 12 months. Patients without anti-TNF-α treatment are evaluated every year. At baseline, just below 600 participants were participating in the GLAS cohort.