Ankylosing spondylitis (AS) is a chronic inflammatory disease that involves the sacroiliac joints, axial skeleton, entheses and peripheral joints. Current therapy for AS is mainly NSAIDs and physiotherapy which are oft insufficient. Treatment with the TNF-alpha blocking agent infliximab was shown to have definite clinical efficacy in patients with active AS on a short- and a long-term-basis over 2 years. We want to show that treatment with infliximab on a long-term basis over 4 years is safe and efficient and can prevent radiographic progression over a long period of time. Further we want to learn about the outcome after discontinuation of anti-TNF-alpha therapy.