To systematically evaluate the efficacy of antibacterial therapy in ulcerative colitis, we carried out a meta-analysis of controlled clinical trials. Within the time period 1966 through September 2006, PUBMED, EMBASE, and SCOPUS were searched for clinical trial studies that investigated the efficacy of antibiotics in ulcerative colitis. We considered clinical remission as our key outcome of interest. Of 122 studies, 10 randomized placebo-controlled clinical trials matched our criteria and were included in the analysis (530 patients). All the studies used antibiotics as an adjunct therapy to conventional treatment of ulcerative colitis (i.e., corticosteroids and 5-aminosalycilic acid). Pooling of these trials yielded odds ratio (OR) of 2.14 (95% confidence interval [CI], 1.48-3.09; P<0.0001) in favor of antimicrobial therapy. Meta-analysis of short-term trials (5-14 days) showed a higher rate of clinical remission in patients treated with antibiotics (OR, 2.02; 95% CI, 1.36-3). These results suggest that adjunctive antibacterial therapy is effective for induction of clinical remission in ulcerative colitis.