This dataset contains clinical data of patients enrolled in the Groningen Leeuwarden Axial Spondyloarthritis (GLAS) cohort, a two-centre prospective observational cohort of patients >18 years of age who have been diagnosed with axial spondyloarthritis. All GLAS participants fulfill the modified New York criteria for ankylosing spondylitis (included since 2004) and/or the ASAS classification criteria for axial spondyloarthritis (included since 2009). Follow-up takes place at regular intervals with data being collected according to a fixed protocol. The 182 patients enrolled in this study were recruited at either of the GLAS clinics (dpt. of Rheumatology and Clinical Immunology, University Medical Centre Groningen dpt. of Rheumatology, Medical Centre Leeuwarden) during a 6-month period (April through September 2019). This dataset contains a combination of baseline data, data collected during standardized follow-up, and one-off data collected specifically for the linked article and another manuscript which is currently in preparation. 1) Baseline data includes date of birth, sex, year of diagnosis and symptom start, history of peripheral and extra-articular manifestations (EAMs), and HLA-B27 status. 2) Standardized follow-up data includes time since inclusion in GLAS, questionnaire data (BASDAI, ASDAS-CRP, ASQoL, BASFI, several assorted questions), data collected by the clinician (anthropometric data, physical function tests, SJC, TJC, MASES, EAMs), lab values (CRP, ESR), medication use (bDMARDs, NSAIDs, opiates, antidepressants, anticonvulsants), and current classification (ankylosing spondylitis vs. non-radiographic axial spondyloarthritis). 3) One-off data collection includes the Central Sensitization Inventory (CS), Revised Illness Perception Questionnaire (IPQ-R), Pain Catastrophizing Scale (PCS), and the highest completed level of education. Participants in this dataset have been pseudonimized. Only the GLAS PI, researchers and directly involved clinicians have access to the key.