The Division of Rheumatology at University of California San Francisco is conducting a research study on the treatment of ankylosing spondylitis (AS) with a new therapy currently used for people with other forms of arthritis. The drug, called Enbrel (or etanercept), is a protein that is given twice weekly by injection underneath the skin. It blocks the action of tumor necrosis factor-alpha (TNF-alpha), a substance that may be involved in AS, rheumatoid arthritis, and other inflammatory conditions. We will randomly assign patients to receive either the drug or a placebo (inactive treatment) for 4 months. The results we will monitor include morning stiffness, spinal mobility, activities of daily life, and safety of the drug.