This study is aimed at investigation of efficacy and safety of ustekinumab (monoclonal antibody against interleukin 12 and 23) treatment in patients with active ankylosing spondylitis (AS) fulfilling the modified New York criteria who have had an inadequate response to standard therapy with non-steroidal anti-inflammatory drugs (NSAIDs) or do not tolerate or have a contraindication for NSAIDs.