There is a great individual variability of pharmacokinetic of infliximab in patients treated for ankylosing spondylitis (AS). Moreover, some patients have a treatment with methotrexate (MTX), others have not.||Thus, the aim of this study is to assess the effect of MTX on the pharmacokinetic properties of infliximab in 30 patients treated for AS, 15 with MTX, 15 without.