Background: Crohn's disease (CD) is a severe, lifelong disease characterised by inflammation of the gastrointestinal mucosa. The impact on patients and society is high as ill health can be lifelong and can negatively affect patients' quality of life. Costs to the NHS are high, particularly for patients needing hospitalisation. Conventional treatment pathways are complex. More recently, a group of drugs called tumour necrosis factor (TNF) inhibitors (anti-TNF-α agents) have been evaluated for their effectiveness in CD. One of these, infliximab, is currently recommended by the National Institute for Health and Clinical Excellence (NICE; 2002) for patients with severe, active CD where patients are refractory to or intolerant of conventional treatment. Objectives: To investigate whether there is evidence for greater clinical effectiveness or cost-effectiveness for either adalimumab or infliximab. Data sources: Cochrane Library (Cochrane Central Register of Controlled Trials) 2007 Issue 2; MEDLINE (Ovid) 2000 to May/June 2007; MEDLINE In-Process & Other Non- Indexed Citations (Ovid) 4 June and 26 June 2007; EMBASE (Ovid) 2000 to May/June 2007. The European Medicines Agency, the US Food and Drug Administration and other relevant websites. Review methods: Standard systematic review methods were used for study identification and selection, data extraction and quality assessment. Only randomised controlled trials (RCTs) comparing adalimumab or infliximab with standard treatment (placebo), RCTs comparing adalimumab with infliximab, or RCTs comparing different dosing regimens of either adalimumab or infliximab in adults and children with moderate-to-severe active CD intolerant or resistant to conventional treatment were eligible for inclusion. A systematic review of published studies on the cost and cost-effectiveness of adalimumab and infliximab was undertaken. The economic models of cost-effectiveness submitted by the manufacturers of both drugs were critically appraised and, where appropriate, rerun using parameter inputs based on the evidence identified by the authors of the technology asessment report. A de novo Markov state transition model was constructed to calculate the incremental cost-effectiveness ratio for adalimumab and infliximab therapy compared with standard care. Results: Based on 11 trials, there was evidence from both induction and maintenance trials that both adalimumab and infliximab therapy were beneficial compared with placebo (standard care) for adults with moderate-to-severe CD and, for infliximab, for adults with fistulising CD; results were statistically significant for some time points. Between 6% and 24% (adalimumab), and 21% and 44% (infliximab) more patients achieved remission with anti-TNF-α antibodies than with placebo in the induction trials. Between 24% and 29% (adalimumab), and 14% and 24% (infliximab) more patients achieved remission with anti- TNF-α antibodies in the two large maintenance trials at reported follow-up. In fistulising CD, between 29% and 42% (induction trial) and 23% (maintenance trial) more patients achieved a > 50% reduction in fistulas with infliximab than with placebo at reported followup. There was no direct evidence to show that 'responders' were more likely to benefit from treatment than 'non-responders' in the longer term. Few differences were found between treatment and standard care arms for selected adverse events, though high proportions of scheduled crossovers resulted in a lack of a true placebo group in most of the maintenance trials. No published studies on the cost-effectiveness of adalimumab were identified. The four independently funded studies identified for infliximab suggested high costeffectiveness ratios [all above £50,000/quality-adjusted life-year (QALY) for non-fistulising disease and all above £100,000/QALY for fistulising disease]. A budget impact assessment suggested that total cost to the NHS in England and Wales for induction in severe disease only could range between £17M and £92M and for maintenance for 1 year between £140M and £200M. Limitations: Regarding clinical effectiveness, there were concerns about the trial design and lack of clarity, which may have affected interpretation of results. None of the trials matched exactly the licence indications or NICE guidance, which specify the use of these drugs in patients with 'severe' disease. All trials were multicentre, and applicability to UK populations, particularly in terms of standard care being provided and in terms of patients having failed or having become intolerant to conventional treatment, was uncertain. The published economic models relied heavily on little information and data from small samples. Conclusions: Anti-TNF therapy with adalimumab or infliximab may have a beneficial effect compared with standard care on outcome measures for induction and maintenance. The findings were that for induction, both adalimumab and infliximab are cost-effective (dominant relative to standard care) in the management of severe CD, and adalimumab (but not infliximab) is cost-effective for moderate CD, according to limits generally accepted by NICE. On the basis of the analysis presented here, neither drug is likely to be cost-effective as maintenance therapy for moderate or severe disease. Perhaps, most importantly, the analysis reflected the fact that a substantial number of patients would achieve remission under standard care and that the incidence of relapse among those in remission was such that maintenance therapy would have to show greater effectiveness than at present and/ or be much less costly than it currently is in order to reach the levels of generally accepted cost-effectiveness. Any future trials need to be designed to meet the particular challenges of measuring and quantifying benefit in this patient group. Funding: The research was funded by the HTA programme on behalf of NICE.