Background: TNF (tumor necrosis factor)-alpha inhibitors block a key protein in the inflammatory chain reaction responsible for joint inflammation, pain, and damage in ankylosing spondylitis. Objectives: To assess the benefit and harms of adalimumab, etanercept, golimumab, and infliximab (TNF-alpha inhibitors) in people with ankylosing spondylitis. Search methods: We searched the following databases to January 26, 2009: MEDLINE (from 1966); EMBASE (from 1980); the Cochrane Central Register of Controlled Trials (CENTRAL; 2008, Issue 4); ACP Journal Club; CINAHL (from 1982); and ISI Web of Knowledge (from 1900). We ran updated searches in May 2012, October 2013, and in June 2014 for McMaster PLUS. We searched major regulatory agencies for safety warnings and clinicaltrials.gov for registered trials. Selection criteria: Randomized controlled trials (RCTs) comparing adalimumab, etanercept, golimumab and infliximab to placebo, other drugs or usual care in patients with ankylosing spondylitis, reported in abstract or full-text. Data collection and analysis: Two authors independently assessed search results, risk of bias, and extracted data. We conducted Bayesian mixed treatment comparison (MTC) meta-analyses using WinBUGS software. To investigate a class-effect of harms across biologics, we pooled harms data using Review Manager 5. Main results: We included twenty-one, short-term (24 weeks or less) RCTs with a total of 3308 participants; 18 contributed data to the MTC analysis: adalimumab (4 studies), etanercept (8 studies), golimumab (2 studies), infliximab (3 studies), and one head-to-head study (etanercept versus infliximab) which was unblinded and considered at a higher risk of bias. The risk of selection and detection bias was low or unclear for most of the studies. The risk of selective outcome reporting was low for most studies as they reported on outcomes recommended by the Assessment of SpondyloArthritis international Society. We found little heterogeneity and no significant inconsistency in the MTC analyses. The majority of the studies were funded by pharmaceutical companies. Most studies permitted concomitant therapy of stable doses of disease-modifying anti-rheumatic drugs, non-steroidal anti-inflammatory drugs, or corticosteroids, but allowances varied across studies. Compared with placebo, there was high quality evidence that patients on an anti-TNF agent were three to four times more likely to achieve an ASAS40 response (assessing spinal pain, function, and inflammation, as measured by the mean of intensity and duration of morning stiffness, and patient global assessment) by six months (adalimumab: risk ratio (RR) 3.53, 95% credible interval (Crl) 2.49 to 4.91; etanercept: RR 3.31, 95% Crl 2.38 to 4.53; golimumab: RR 2.90, 95% Crl 1.90 to 4.23; infliximab: RR 4.07, 95% Crl 2.80 to 5.74, with a 25% to 40% absolute difference between treatment and placebo groups. The number needed to treat (NNT) to achieve an ASAS 40 response ranged from 3 to 5. There was high quality evidence of improvement in physical function on a 0 to 10 scale (adalimumab: mean difference (MD) -1.6, 95% Crl -2.2 to -0.9; etanercept: MD -1.1, 95% CrI -1.6 to -0.6; golimumab: MD -1.5, 95% Crl -2.3 to -0.7; infliximab: MD -2.1, 95% Crl -2.7 to -1.4, with an 11% to 21% absolute difference between treatment and placebo groups. The NNT to achieve the minimally clinically important difference of 0.7 points ranged from 2 to 4. Compared with placebo, there was moderate quality evidence (downgraded for imprecision) that patients on an anti-TNF agent were more likely to achieve an ASAS partial remission by six months (adalimumab: RR 6.28, 95% Crl 3.13 to 12.78; etanercept: RR 4.24, 95% Crl 2.31 to 8.09; golimumab: RR 5.18, 95% Crl 1.90 to 14.79; infliximab: RR 15.41, 95% Crl 5.09 to 47.98 with a 10% to 44% absolute difference between treatment and placebo groups. The NNT to achieve an ASAS partial remission response ranged from 3 to 11. There was low to moderate level evidence of a greater reduction in spinal inflammation as measured by magnetic resonance imaging though the absolute differences were small and the clinical relevance of the difference was unclear: adalimumab (1 trial; -6% (95% confidence interval (CI) -12% to 0.05%); 1 trial: 53.6% mean decrease from baseline versus 9.4% mean increase in the placebo group), golimumab (1 trial; -2.5%, (95% CI -5.6% to -0.7%)), and infliximab (1 trial; -3% (95% CI -4% to -2.4%)). Radiographic progression was measured in one trial (N = 60) of etanercept versus placebo and it found that radiologic changes were similar in both groups (detailed data not provided). There were few events of withdrawals due to adverse events leading to imprecision around the estimates. When all the anti-TNF agents were combined against placebo, there was moderate quality evidence from 16 studies of an increased risk of withdrawals due to adverse events in the anti-TNF group (Peto odds ratio (OR) 2.44, 95% CI 1.26 to 4.72; total events: 38/1637 in biologic group; 7/986 in placebo) though the absolute increase in harm was small (1%; 95% CI 0% to 2%). Due to low event rates, evidence of the effect of individual TNF-inhibitors against placebo or for all four biologics pooled together versus placebo on serious adverse events is inconclusive (moderate quality; downgraded for imprecision). For all anti-TNF pooled versus placebo based on 16 studies: Peto OR 1.45, 95% CI 0.85 to 2.48; 51/1530 in biologic group; 18/878 in placebo; absolute difference: 1% (95% CI 0% to 2%). Using indirect comparison methodology, and one head-to-head study of etanercept versus infliximab, wide confidence intervals meant that results were inconclusive for evidence of differences in the major outcomes between different anti-TNF agents. Regulatory agencies have published warnings about rare adverse events of serious infections, including tuberculosis, malignancies and lymphoma. Authors' conclusions: There is moderate to high quality evidence that anti-TNF agents improve clinical symptoms in the treatment of ankylosing spondylitis. More participants withdrew due to adverse events when on an anti-TNF agent but we did not find evidence of an increase in serious adverse events, though event rates were low and trials had a short duration. The short-term toxicity profile appears acceptable. Based on indirect comparison methodology, we are uncertain whether there are differences between anti-TNF agents in terms of the key benefit or harm outcomes.