Adults with active ankylosing spondylitis (AS) were randomized in a 2:1 ratio to receive treatment with adalimumab 40 mg every other week (eow) or matching placebo, given subcutaneously (SC), in the 12-week double-blind (DB) phase. Randomized participants received one SC injection of the appropriate DB study medication (adalimumab 40 mg or matching placebo) at Week 0 and then eow until Week 10. Participants who completed the DB phase could enter the 12-week open-label (OL) phase, during which all participants received treatment with adalimumab 40 mg eow, starting at Weeks 12 through 22. No study drug was administered or injected at the final study visit (Week 24). A follow-up visit occurred 70 days after the last dose of study drug (in DB or OL phases) to obtain information on any ongoing or new adverse events (AEs).