Abstract Background The aim of our study was to investigate the influence of conventional synthetic disease-modifying anti-rheumatic drugs (csDMARDs) and body mass index (BMI) on circulating drug levels and clinical response to tumour necrosis factor inhibitor (TNFi) therapy in axial spondyloarthritis (axSpA) patients. Methods Prospective observational study during 1 year with 2 cohorts (Madrid and Amsterdam) including 180 axSpA patients treated with standard doses of infliximab or adalimumab. Patients were stratified by BMI, being 78 (43%) normal weight (18.5–24.9 kg/m2) and 102 (57%) overweight/obese (≥ 25.0 kg/m2). After the first year of treatment, TNFi trough levels were measured by capture ELISA. Clinical response to TNFi was defined as ∆BASDAI ≥ 2 and clinical remission as BASDAI