Objective. Obesity can be a factor that affects response to anti-TNF drugs. However, studies on patients with ankylosing spondylitis (AS) are rare. We aimed to determine whether obesity affects serum levels of adalimumab (ADL), and immunogenicity and clinical efficacy of the drug in patients with AS. Methods. A cross-sectional study on 57 patients with axial AS receiving ADL was conducted. They received DMARD per standard of care at their rheumatologist's discretion. Patients' body mass index (BMI) was obtained when ADL treatment began. Clinical response was evaluated using the Spanish versions of the BASDAI index and the ASDAS ESR index. Serum concentrations of free ADL (trough level) and anti-ADL antibodies were measured using Promonitor-ADL and Promonitor Anti-ADL ELISA kits (Progenika Grifols SA, Spain), just prior to the next subcutaneous injection of ADL. Results. Patients with BMI >30 kgl nt2 (obese) as opposed to BMI <25 kg1 m2 (normal), presented lower blood ADL levels [5.0 (5.52) vs. 9.14 (4.3), p=0.032.1, increased ASDAS scores (2.58 [0.79] vs. 1.9 [0.83], p=0.03), and shorter ADL treatment time: 1.01 [0.84] vs. (1.85 [1.65]; p=0.081), and increased BASDAI results (5.04 [2.5] vs. 3.5 [1.88]; p=0.06). Obese patients showed a lower probability of 'clinical response to ADL versus non-obese patients with regard to achieving BASDAI 54 (OR.- 3.5, 95%Cl: 0.84-17.19; p=0.05) or ASDAS.s2.1 (OR: 4.64, 95%Cl: 1.02-24.13; p=0.02). Conclusion. Of the AS patients receiving treatment with ADL, those that are obese had significantly lower serum ADL levels and decreased clinical response without an increase in immunogenicity.