In this Phase II clinical trial we will use tumor necrosis factor receptor p75 fusion protein (TNFR:Fc, or etanercept) to treat patients with ankylosing spondylitis (AS). TNFR:Fc is an antagonist of tumor necrosis factor (TNF), a cytokine that researchers have shown to play a possible role in disease pathogenesis of ankylosing spondylitis, rheumatoid arthritis, and vasculitis, as well as other inflammatory conditions.