This randomized, double-blind, placebo-controlled study will evaluate the safety and efficacy of RoActemra/Actemra (tocilizumab) in patients with ankylosing spondylitis (AS) who have failed treatment with non-steroidal anti-inflammatory drugs and are naïve to tumor necrsos factor (TNF) antagonist therapy. In Part 1 of the study, patients will be randomized to receive either RoActemra/Actemra 8 mg/kg intravenously (IV) or placebo every 4 weeks for 12 weeks. In Part 2, patients will be randomized to receive RoActemra at either 8 mg/kg or 4 mg/kg IV or placebo every 4 weeks for 24 weeks. The double-blind treatment period will be followed by open-label treatment with RoActemra/Actemra 8 mg/kg iv every 4 weeks until Week 208 for all patients. Anticipated time on study treatment is 208 weeks.