BACKGROUND We assessed the tolerability and clinical benefit of adalimumab, a human antibody to tumor necrosis factor (TNF), in patients with Crohn's disease who had previously received and responded to the chimeric anti-TNF antibody infliximab, but who no longer had a sustained response and/or tolerance to infliximab. METHODS A total of 24 patients with Crohn's disease who had lost responsiveness or developed intolerance (acute or delayed infusion reactions) to infliximab were enrolled in a 12-wk uncontrolled trial and treated with subcutaneous adalimumab 80 mg at week 0 and then 40 mg every other week starting at week 2. After week 4, the dose could be escalated to 40 mg weekly in patients who did not achieve clinical remission, complete fistula closure, and complete steroid withdrawal. Outcome measures included the ability to tolerate adalimumab and clinical remission (defined as a Crohn's disease activity index (CDAI) score < or =150 points) and clinical response (defined as a decrease in the CDAI) > or =100 points) in patients who had a baseline CDAI score > or =220. RESULTS None of the patients experienced acute or delayed hypersensitivity reactions during treatment with adalimumab (including 14 who previously experienced treatment-limiting acute hypersensitivity reactions and 6 who previously experienced delayed hypersensitivity reactions with infliximab). Of 17 patients with baseline CDAI scores > or =220: clinical remission occurred at weeks 4 and 12 in 2 (12%) and 5 (29%), respectively; and clinical response occurred in 7 (41%) and 10 (59%), respectively. Nineteen patients (79%) escalated their dose during weeks 4-6. CONCLUSIONS Adalimumab is well tolerated and appears to be a clinically beneficial option for patients with Crohn's disease who have previously lost their response to, or cannot tolerate infliximab.