Recent observations from animal and human studies suggest that tumor necrosis factor-alpha (TNF-alpha) may play a role in disease activity in AS and other seronegative spondyloarthropathies. This study aims to test the efficacy of TNFR:Fc used in conjunction with standard medications in the treatment of AS. We will give patients either 25mg of TNFR:Fc or placebo subcutaneously twice a week for 4 months. Outcome measures will include measures of function, pain, morning stiffness, patient global assessment, and swollen joint count, as well as safety measures.